Steel sheet for carburizing, and method for manufacturing steel sheet for carburizing

ABSTRACT

[Object] To provide a steel sheet for carburizing having further improved formability and toughness after carburizing, and a method for manufacturing the same.[Solution] A steel sheet consisting of, in mass %, C: more than or equal to 0.02%, and less than 0.30%, Si: more than or equal to 0.005%, and less than or equal to 0.5%, Mn: more than or equal to 0.01%, and less than or equal to 3.0%, P: less than or equal to 0.1%, S: less than or equal to 0.1%, sol. Al: more than or equal to 0.0002%, and less than or equal to 3.0%, N: more than or equal to 0.0001, and less than or equal to 0.035%, and the balance: Fe and impurities, in which average crystal grain size of ferrite is smaller than 10 μm, average equivalent circle diameter of carbide is 5.0 μm or smaller, percentage of number of carbides with an aspect ratio of 2.0 or smaller is 80% or larger relative to the total carbides, percentage of number of carbides present in ferrite crystal grain is 60% or larger relative to the total carbides, and average nitrogen concentration in a region ranging from topmost surface of steel sheet to a depth of 50 μm is 0.040 mass % or higher and 0.200 mass % or lower.

TECHNICAL FIELD

The present invention relates to a steel sheet for carburizing, and amethod for manufacturing the steel sheet for carburizing.

BACKGROUND ART

In recent years, mechanical and structural parts such as automotivegear, clutch plate and damper have been required to be highly durable,and in addition to be manufacturable at low costs. These parts havewidely been manufactured by cutting and carburizing using hot-forgedmaterials. However, in response to increasing need for cost reduction,having been developed are technologies by which hot-rolled steel sheetor cold-rolled steel sheet, employed as a starting material, iscold-worked into shapes of the parts, followed by carburizing. In thecold-working, components are formed by punching materials, followed bybending, drawing, hole expansion or the like. In this process, a steelsheet for carburizing to be worked is required to have good bendabilitywhich relates to a most basic deformation mode. In addition, automotivecomponents such as damper for torque converter are required to haveexcellent impact resistant characteristics including toughness. Fromthis point of view, a variety of technologies have recently beenproposed.

For example, Patent Literature 1 listed below proposes a technology forforming a structure of a hot-rolled steel sheet with ferrite andpearlite, and then spherodizing carbide by spherodizing annealing.

Meanwhile, Patent Literature 2 listed below proposes a technology forimproving impact characteristics of a carburized member, by controllingparticle size of carbide, as well as controlling percentage of thenumber of carbides at ferrite crystal grain boundaries relative to thenumber of carbides within ferrite particles, and further by controllingcrystal size of the ferrite matrix.

Moreover, Patent Literature 3 listed below proposes a technology forimproving cold workability, by controlling particle size and aspectratio of carbide, as well as controlling crystal size of ferrite matrix,and further by controlling aspect ratio of ferrite.

CITATION LIST Patent Literature

Patent Literature 1: JP 3094856B

Patent Literature 2: WO 2016/190370

Patent Literature 3: WO 2016/148037

SUMMARY OF INVENTION Technical Problem

The aforementioned mechanical and structural parts are required to behardenable for enhanced strength. That is, the materials used formechanical and structural parts are required to satisfy formability,while keeping hardenability. In addition, the mechanical and structuralparts after carburized are required to have impact resistancecharacteristics (particularly, toughness after carburizing).

With the manufacturing method disclosed in Patent Literature 1, mainlyrelying upon control of a microstructure of carbide, is however notexpected to effectively improve the toughness after carburizing,although the method might improve impact resistance characteristicsoriginated from cracks that may be introduced during the cold-working.Meanwhile, the manufacturing method proposed in Patent Literature 2,mainly relying upon control of microstructures of carbide and ferrite,might improve the formability, but still have room for improvement inpursuit of more advanced toughness, if intended to be applied tospecific automotive components such as damper for automotive torqueconverter, for which a high level of impact resistance is required.Furthermore, use of the technology proposed in Patent Literature 3 mightimprove the formability, but still have room for improvement in pursuitof more advanced toughness, if intended to be applied to specificautomotive components such as damper of automotive torque converter, forwhich a high level of impact resistance is required. As described above,the ever-proposed technologies still have room for improvement in aneffort to obtain a sufficient level of toughness after carburizing,while keeping formability and hardenability of the steel sheet forcarburizing. Hence, there has been desired the steel sheet forcarburizing, which is more suitably applicable to specific automotivecomponents such as damper of torque converter, for which a high level ofimpact resistance is required.

The present invention was made in consideration of the aforementionedproblems, an object of which is to provide a steel sheet for carburizingfurther improved in the formability, and toughness after carburizing,and a method for manufacturing the same.

Solution to Problem

The present inventors made thorough investigations into methods ofsolving the aforementioned problems, and consequently reached an ideathat, as detailed later, the formability during cold-working and thetoughness after carburizing may be improved while keeping thehardenability, by appropriately controlling position of production ofcarbides in ferrite crystal grain, and nitrogen concentration in a skinlayer of the steel sheet, to complete the present invention.

Summary of the present invention reached on the basis of such idea is asfollows.

-   [1]

A steel sheet for carburizing consisting of, in mass %,

-   C: more than or equal to 0.02%, and less than 0.30%,-   Si: more than or equal to 0.005%, and less than or equal to 0.5%,-   Mn: more than or equal to 0.01%, and less than or equal to 3.0%,-   P: less than or equal to 0.1%,-   S: less than or equal to 0.1%,-   sol. Al: more than or equal to 0.0002%, and less than or equal to    3.0%,-   N: more than or equal to 0.0001, and less than or equal to 0.035%,    and-   the balance: Fe and impurities,

in which average crystal grain size of ferrite is smaller than 10 μm,

average equivalent circle diameter of carbide is 5.0 μm or smaller,

percentage of number of carbides with an aspect ratio of 2.0 or smalleris 80% or larger relative to the total carbides,

percentage of number of carbides present in ferrite crystal grain is 60%or larger relative to the total carbides, and

average nitrogen concentration in a region ranging from topmost surfaceof steel sheet to a depth of 50 μm is 0.040 mass % or higher and 0.200mass % or lower.

-   [2]

The steel sheet for carburizing according to [1], further including, inplace of part of the balance Fe, one of, or two or more of, in mass %,

-   Cr: more than or equal to 0.005%, and less than or equal to 3.0%,-   Mo: more than or equal to 0.005%, and less than or equal to 1.0%,-   Ni: more than or equal to 0.010%, and less than or equal to 3.0%,-   Cu: more than or equal to 0.001%, and less than or equal to 2.0%,-   Co: more than or equal to 0.001%, and less than or equal to 2.0%,-   Nb: more than or equal to 0.010%, and less than or equal to 0.150%,-   Ti: more than or equal to 0.010%, and less than or equal to 0.150%,-   V: more than or equal to 0.0005%, and less than or equal to 1.0%,    and-   B: more than or equal to 0.0005%, and less than or equal to 0.01%.-   [3]

The steel sheet for carburizing according to [1] or [2], furtherincluding, in place of part of the balance Fe, at least either one of,in mass %,

-   W: less than or equal to 1.0%, or-   Ca: less than or equal to 0.01%.-   [4]

A method for manufacturing the steel sheet for carburizing according toany one of [1] to [3], the method including:

a hot-rolling step, in which a steel material having the chemicalcomposition according to any one of [1] to [3] is heated, hot finishrolling is terminated in a temperature range of 800° C. or higher andlower than 920° C., followed by winding at a temperature of 700° C. orlower; and

an annealing step, in which the steel sheet obtained by the hot-rollingstep, or, the steel sheet having been cold-rolled subsequently to thehot-rolling step is heated in an atmosphere with nitrogen concentrationcontrolled to 25% or higher in volume fraction, at an average heatingrate of 5° C./h or higher and 100° C./h or lower, up into a temperaturerange not higher than point Ac₁ defined by equation (1) below, annealedin the temperature range not higher than the point Ac₁ for 10 h orlonger and 100 h or shorter, and then cooled at an average cooling rateof 5° C./h or higher and 100° C./h or lower in a temperature range froma temperature at the end of annealing down to 550° C.,

in the hot-rolling step, cooling being started within one second afterend of the hot finish rolling, at an average cooling rate of higher than50° C./s, and

an average grain size of ferrite after the annealing being controlled tosmaller than 10 μm.

-   [5] A method for manufacturing the steel sheet for carburizing    according to [4], which further includes a continuous casting step    for obtaining the steel material to be subjected to the hot-rolling    step, in which at least either soundness enhancing treatment of the    steel material, namely production of a predetermined inclusion, or    reduction of center segregation of a predetermined element, is    carried out.

$\begin{matrix}\left\lbrack {{Math}.\mspace{14mu} 1} \right\rbrack & \; \\{{Ac}_{1} = {750.8 - {26.6\lbrack C\rbrack} + {17.6\lbrack{Si}\rbrack} - {11.6\lbrack{Mn}\rbrack} - {22.9\lbrack{Cu}\rbrack} - {23\lbrack{Ni}\rbrack} + {24.1\lbrack{Cr}\rbrack} + {22.5\lbrack{Mo}\rbrack} - {39.7\lbrack V\rbrack} - {5.7\lbrack{Ti}\rbrack} + {232.4\lbrack{Nb}\rbrack} - {169.4\lbrack{Al}\rbrack} - {894.7\lbrack B\rbrack}}} & {{Equation}\mspace{14mu} (1)}\end{matrix}$

In equation (1) above, notation [X] represents the content of element X(in mass %), which is substituted by zero if such element X is absent.

Advantageous Effects of Invention

As explained above, according to the present invention, it now becomespossible to provide a steel sheet for carburizing having furtherimproved formability and toughness after carburizing.

DESCRIPTION OF EMBODIMENTS

Preferred embodiments of the present invention will be detailed below.

(Details of Examination Made by Present Inventors, and Reached Idea)

Prior to description on the steel sheet for carburizing and the methodfor manufacturing the same according to the present invention, theexamination made by the present inventors, aimed at solving theaforementioned problems, will be detailed below.

In the examination, the present inventors started first by examining amethod for improving formability (particularly, bendability) beforecarburizing.

In order to improve the formability (particularly, bendability) beforecarburizing, it is important to suppress cracking during bendingdeformation, and further to suppress, if the cracking once occurred,propagation of the produced crack. Control of the aspect ratio (longaxis/short axis) of carbide produced in the steel sheet is effective tosuppress the cracking, posing importance of reduction of the aspectratio of carbide by spherodizing annealing. Meanwhile, suppression ofproduction of coarse carbide, and control of position of precipitationof carbide are effective to suppress propagation of the crack. That is,since carbide produced in the grain boundary of ferrite can promote thecrack to propagate while routed through the grain boundary, so that itis important to produce carbide inside crystal grains of ferrite. Suchpropagation of crack through the grain boundary is considered to besuppressed by producing carbide inside the crystal grains of ferrite.

After employing such structural control, the present inventors furtherfocused on improvement of toughness through condensation of nitrogen ina skin layer of the steel sheet for carburizing, in pursuit of a methodfor improving the impact resistance characteristics after carburizing,and made thorough investigations and researches on operations andeffects of the nitrogen condensation in the skin layer of the steelsheet. The present inventors consequently found that the toughness aftercarburizing (particularly, impact value at room temperature) may bedramatically improved by controlling the nitrogen concentration in theskin layer of the steel sheet. More specifically, it was found thatnitrogen, in the process of annealing a hot-rolled steel sheet or acold-rolled steel sheet, was successfully condensed in the skin layer ofthe steel sheet for carburizing, by controlling the nitrogenconcentration in an atmosphere at a level not lower than a predeterminedthreshold value, and that the impact value at room temperature ofcarburized member made of the steel sheet for carburizing wasdramatically improved as a consequence.

A possible mechanism of improvement of toughness after carburizing is asfollows. By annealing the steel sheet in a nitrogen-rich atmosphere, theatmospheric nitrogen enters the steel sheet to form nitride in the skinlayer of the steel sheet. The thus formed nitride is mainly composed offine AIN, and can demonstrate an effect of suppressing growth of grainsof prior austenite during carburizing heat treatment. Grain size ofprior austenite and grain size of transformed martensite are in aproportional relationship. It is therefore contemplated that if thegrains of prior austenite are suppressed from growing by such fine AlN,also the grain size of martensite in a structure of carburized memberwill be micronized, and the impact value dramatically increased as aconsequence. Extensive investigations by the present inventors revealedthat fine AIN was found to be produced in the skin layer of the steelsheet for carburizing, resulted in increase of impact value in thecarburized member.

Note that the aforementioned bendability and toughness after carburizingwill become inferior as the strength of steel sheet increases. Meanwhilefrom the viewpoint of satisfying a necessary level of hardenability forthe steel sheet for carburizing, the steel sheet is desired to bestrengthened. In order to balance these contradictory characteristics,the key is to satisfy the hardenability by way of the above-outlinedstructural control, as well as to improve the bendability and toughnessafter carburizing. Hence, through the above-outlined structural control,obtainable is the steel sheet for carburizing that is well balancedamong the hardenability, bendability, and toughness after carburizing.

The present inventors have succeeded, by the aforementioned structuralcontrol of steel sheet, in improving the bendability during cold-workingand the toughness after carburizing, while keeping the hardenability. Inthis way, it now becomes possible to obtain the steel sheet forcarburizing well balanced among the hardenability, formability, andtoughness after carburizing.

The steel sheet for carburizing and the method for manufacturing thesame according to embodiments of the present invention, as detailedlater, have been reached on the basis of the aforementioned findings.Paragraphs below will detail the steel sheet for carburizing and themethod for manufacturing the same according to the embodiments reachedon the basis of the findings.

(Steel Sheet for Carburizing)

First, the steel sheet for carburizing according to the embodiment ofthe present invention will be detailed.

The steel sheet for carburizing according to the embodiment has apredetermined chemical composition detailed below. In addition, thesteel sheet for carburizing according to this embodiment has a specificmicrostructure featured by that the average equivalent circle diameterof carbide is 5.0 μm or smaller; that the percentage of the number ofcarbides with an aspect ratio of 2.0 or smaller is 80% or largerrelative to the total carbides; that the percentage of the number ofcarbides present in ferrite crystal grain is 60% or larger relative tothe total carbides; and that the nitrogen concentration in a regionranging from the topmost surface of the steel sheet to a depth of 50 μmis 0.040 mass % or higher and 0.200 mass % or lower. Hence the steelsheet for carburizing according to this embodiment will have furtherimproved formability and toughness after carburizing, while keeping thehardenability.

<Chemical Composition of Steel Sheet for Carburizing>

First, chemical ingredients at the middle-thickness portion of the steelsheet for carburizing according to this embodiment will be detailed.Note that in the following description, notation “%” relevant to thechemical components means “mass %”, unless otherwise specifically noted.

-   [C: More than or Equal to 0.02%, and Less than 0.30%]

C (carbon) is an element necessary for keeping strength at the center ofthickness of a finally obtainable carburized member. In the steel sheetfor carburizing, C is also an element solid-soluted into the grainboundary of ferrite to enhance the strength of the grain boundary, tothereby contribute to improvement of the bendability.

With the content of C less than 0.02%, the aforementioned effect ofimproving the bendability will not be obtained. Hence the content of Cin the steel sheet for carburizing according to the embodiment isspecified to be more than or equal to 0.02%. The content of C ispreferably more than or equal to 0.05%. Meanwhile, with the content of Cmore than or equal to 0.30%, carbide will have an average equivalentcircle diameter exceeding 5.0 μm, thereby the bendability will degrade.Hence the content of C in the steel sheet for carburizing according tothe embodiment is specified to be less than 0.30%. The content of C ispreferably less than or equal to 0.20%. Note that, taking a balancebetween bendability and hardenability into account, the content of C isfurther preferably be less than or equal to 0.10%.

[Si: More than or Equal to 0.005%, and Less than or Equal to 0.5%]

Si (silicon) is an element that acts to deoxidize molten steel toimprove soundness of the steel. With the content of Si less than 0.005%,the molten steel will not thoroughly be deoxidized. Hence the content ofsilicon in the steel sheet for carburizing according to the embodimentis specified to be more than or equal to 0.005%. The content of Si ispreferably more than or equal to 0.01%. Meanwhile, with the content ofSi more than 0.5%, Si having been solid-soluted in carbide willstabilize the carbide and will allow the carbide to have an averageequivalent circle diameter exceeding 5.0 μm, degrading the bendability.Hence the content of Si in the steel sheet for carburizing according tothe embodiment is specified to be less than or equal to 0.5%. Thecontent of Si is preferably less than or equal to 0.3%.

[Mn: More than or Equal to 0.01%, and Less than or Equal to 3.0%]

Mn (manganese) is an element that acts to deoxidize molten steel toimprove soundness of the steel. With the content of Mn less than 0.01%,the molten steel will not thoroughly be deoxidized. Hence the content ofMn in the steel sheet for carburizing according to the embodiment isspecified to be more than or equal to 0.01%. The content of Mn ispreferably more than or equal to 0.1%. Meanwhile, with the content of Mnmore than 3.0%, Mn having been solid-soluted in carbide will stabilizethe carbide and will allow the carbide to have an average equivalentcircle diameter exceeding 5.0 pm, degrading the bendability. Hence thecontent of Mn is specified to be less than or equal to 3.0. The contentof Mn is more preferably less than or equal to 2.0%, and even morepreferably less than or equal to 1.0%.

[P: Less than or Equal to 0.1%]

P (phosphorus) is an element that segregates in the grain boundary offerrite to degrade the bendability. With the content of P exceeding0.1%, the grain boundary will have considerably reduced strength, andthereby the bendability will degrade. Hence, the content of P in thesteel sheet for carburizing according to the embodiment is specified tobe less than or equal to 0.1%. The content of P is preferably less thanor equal to 0.050%, and more preferably less than or equal to 0.020%.Note that the lower limit of the content of P is not specificallylimited. The content of P reduced below 0.0001% will howeverconsiderably increase cost for dephosphorization, causing economicdisadvantage. Hence the lower limit of content of P will substantiallybe 0.0001% for practical steel sheet.

[S: Less than or Equal to 0.1%]

S (sulfur) is an element that can form an inclusion to degrade thebendability. With the content of S exceeding 0.1%, a coarse inclusionwill be produced, and thereby the bendability will degrade. Hence thecontent of S in the steel sheet for carburizing according to theembodiment is specified to be less than or equal to 0.1%. The content ofS is preferably less than or equal to 0.010%, and more preferably lessthan or equal to 0.008%. Note that the lower limit of content of S isnot specifically limited. The content of S reduced below 0.0005% willhowever considerably increase cost for desulfurization, causing economicdisadvantage. Hence, the lower limit of content of S will substantiallybe 0.0005% for practical steel sheet.

[sol. Al: More than or Equal to 0.0002%, and Less than or Equal to 3.0%]

Al (aluminum) is an element that acts to deoxidize molten steel toimprove soundness of the steel. With the content of Al less than0.0002%, the molten steel will not thoroughly be deoxidized. Hence thecontent of Al (in more detail, the content of sol. Al) in the steelsheet for carburizing according to the embodiment is specified to bemore than or equal to 0.0002%. The content of Al is preferably more thanor equal to 0.0010%, more preferably more than or equal to 0.0050%, andeven more preferably more than or equal to 0.010%. Meanwhile, with thecontent of Al exceeding 3.0%, coarse oxide will be produced, and therebythe bendability will degrade. Hence the content of Al is specified to beless than or equal to 3.0%. The content of Al is preferably less than orequal to 2.5%, more preferably less than or equal to 1.0%, even morepreferably less than or equal to 0.2%, and yet more preferably less thanor equal to 0.05%.

[N: More than or Equal to 0.0001%, and Less than or Equal to 0.035%]

The content of N (nitrogen) in the steel sheet for carburizing accordingto this embodiment need be less than or equal to 0.035%. Note that thecontent of N defined now is an average value of N present throughout thethickness direction of the steel sheet (an average value of the contentof N in the thickness direction). With the content of N exceeding0.035%, a large amount of nitride will be precipitated throughout thethickness direction of the steel sheet for carburizing, making itdifficult to obtain desired bendability. Hence, the content of N in thesteel sheet for carburizing according to the embodiment is specified tobe less than or equal to 0.035%. The content of N is preferably lessthan or equal to 0.030%, more preferably less than or equal to 0.020%,and even more preferably less than or equal to 0.010%. The lower limitof content of N is not specifically limited. The content of N reducedbelow 0.0001% will however considerably increase cost fordenitrification, causing economic disadvantage. Hence, the lower limitof content of N will substantially be 0.0001% for practical steel sheet.Alternatively, in consideration of fully introducing nitrogen into theskin layer of the steel sheet, the content of N may be specified to be0.0020% or larger.

[Cr: More than or Equal to 0.005%, and Less than or Equal to 3.0%]

Cr (chromium) is an element having an effect of increasing thehardenability of the finally obtainable carburized member, and is alsoan element, for the steel sheet for carburizing, having an effect ofmicronizing ferrite crystal grains to further improve the toughnessafter carburizing. Hence in the steel sheet for carburizing according tothe embodiment, Cr may be contained as needed. In order to obtain moreenhanced effect of toughness after carburizing, the content of Cr, ifcontained, is preferably specified to be more than or equal to 0.005%.The content of Cr is more preferably more than or equal to 0.010%.Further, in consideration of the effects of production of carbide andnitride, the content of Cr is preferably less than or equal to 3.0%, inview of obtaining more enhanced effect of toughness after carburizing.The content of Cr is more preferably less than or equal to 2.0%, andeven more preferably less than or equal to 1.6%.

[Mo: More than or Equal to 0.005%, and Less than or Equal to 1.0%]

Mo (molybdenum) is an element having an effect of increasing thehardenability of the finally obtainable carburized member, and is alsoan element, for the steel sheet for carburizing, having an effect ofmicronizing ferrite crystal grains to further improve the toughnessafter carburizing. Hence in the steel sheet for carburizing according tothe embodiment, Mo may be contained as needed. In order to obtain moreenhanced effect of toughness after carburizing, the content of Mo, ifcontained, is preferably specified to be more than or equal to 0.005%.The content of Mo is more preferably more than or equal to 0.010%.Further, in consideration of the effects of production of carbide andnitride, the content of Mo is preferably less than or equal to 1.0%, inview of obtaining more enhanced effect of toughness after carburizing.The content of Mo is more preferably less than or equal to 0.8%.

[Ni: More than or Equal to 0.010%, and Less than or Equal to 3.0%]

Ni (nickel) is an element having an effect of increasing thehardenability of the finally obtainable carburized member, and is alsoan element, for the steel sheet for carburizing, having an effect ofmicronizing ferrite crystal grains to further improve the toughnessafter carburizing. Hence in the steel sheet for carburizing according tothe embodiment, Ni may be contained as needed. In order to obtain moreenhanced effect of toughness after carburizing, the content of Ni, ifcontained, is preferably specified to be more than or equal to 0.010%.The content of Ni is more preferably more than or equal to 0.050%.Further, in consideration of the effects of segregation of Ni in thegrain boundary of ferrite, the content of Ni is preferably less than orequal to 3.0%, in view of obtaining more enhanced effect of toughnessafter carburizing. The content of Ni is more preferably less than orequal to 2.0%, even more preferably less than or equal to 1.0%, and yetmore preferably less than or equal to 0.5%.

[Cu: More than or Equal to 0.001%, and Less than or Equal to 2.0%]

Cu (copper) is an element having an effect of increasing thehardenability of the finally obtainable carburized member, and is alsoan element, for the steel sheet for carburizing, having an effect ofmicronizing ferrite crystal grains to further improve the toughnessafter carburizing. Hence in the steel sheet for carburizing according tothe embodiment, Cu may be contained as needed. In order to obtain moreenhanced effect of toughness after carburizing, the content of Cu, ifcontained, is preferably specified to be more than or equal to 0.001%.The content of Cu is more preferably more than or equal to 0.010%.Further, in consideration of the effects of segregation of Cu in thegrain boundary of ferrite, the content of Cu is preferably less than orequal to 2.0%, in view of obtaining more enhanced effect of toughnessafter carburizing. The content of Cu is more preferably less than orequal to 0.80%.

[Co: More than or Equal to 0.001%, and Less than or Equal to 2.0%]

Co (cobalt) is an element having an effect of increasing thehardenability of the finally obtainable carburized member, and is alsoan element, for the steel sheet for carburizing, having an effect ofmicronizing crystal grains to further improve the toughness aftercarburizing. Hence in the steel sheet for carburizing according to theembodiment, Co may be contained as needed. In order to obtain moreenhanced effect of toughness after carburizing, the content of Co, ifcontained, is preferably specified to be more than or equal to 0.001%.The content of Co is more preferably more than or equal to 0.010%.Further, in consideration of the effects of segregation of Co in thegrain boundary of ferrite, the content of Co is preferably less than orequal to 2.0%, in view of obtaining more enhanced effect of toughnessafter carburizing. The content of Co is more preferably less than orequal to 0.80%.

[Nb: More than or Equal to 0.010%, and Less than or Equal to 0.150%]

Nb (niobium) is an element that contributes to micronize ferrite crystalgrains to further improve the toughness after carburizing. Hence in thesteel sheet for carburizing according to the embodiment, Nb may becontained as needed. In order to obtain more enhanced effect oftoughness after carburizing, the content of Nb, if contained, ispreferably specified to be more than or equal to 0.010%. The content ofNb is more preferably more than or equal to 0.035% Further, inconsideration of the effects of production of carbide and nitride, thecontent of Nb is preferably less than or equal to 0.150%, in view ofobtaining more enhanced effect of toughness after carburizing. Thecontent of Nb is more preferably less than or equal to 0.120%, even morepreferably less than or equal to 0.100%, and yet more preferably lessthan or equal to 0.050%.

[Ti: More than or Equal to 0.010%, and Less than or Equal to 0.150%]

Ti (titanium) is an element that contributes to micronize ferritecrystal grains to further improve the toughness after carburizing. Hencein the steel sheet for carburizing according to the embodiment, Ti maybe contained as needed. In order to obtain more enhanced effect oftoughness after carburizing, the content of Ti, if contained, ispreferably specified to be more than or equal to 0.010%. The content ofTi is more preferably more than or equal to 0.035% Further, inconsideration of the effects of production of carbide and nitride, thecontent of Ti is preferably less than or equal to 0.150%, in view ofobtaining more enhanced effect of toughness after carburizing. Thecontent of Ti is more preferably less than or equal to 0.120%, even morepreferably less than or equal to 0.050%, and yet more preferably lessthan or equal to 0.020%.

[V: More than or Equal to 0.0005%, and Less than or Equal to 1.0%]

V (vanadium) is an element that contributes to micronize ferrite crystalgrains to further improve the toughness after carburizing. Hence in thesteel sheet for carburizing according to the embodiment, V may becontained as needed. In order to obtain more enhanced effect oftoughness after carburizing, the content of V, if contained, ispreferably specified to be more than or equal to 0.0005%. The content ofV is more preferably more than or equal to 0.0010% Further, inconsideration of the effects of production of carbide and nitride, thecontent of V is preferably less than or equal to 1.0%, in view ofobtaining more enhanced effect of toughness after carburizing. Thecontent of V is more preferably less than or equal to 0.80%.

[B: More than or Equal to 0.0005%, and Less than or Equal to 0.01%]

B (boron) is an element that segregates in the grain boundary of ferriteto enhance strength of the grain boundary, to thereby further improvethe toughness after carburizing. Hence in the steel sheet forcarburizing according to the embodiment, B may be contained as needed.In order to obtain more enhanced effect of toughness after carburizing,the content of B, if added, is preferably specified to be more than orequal to 0.0005%. The content of B is more preferably more than or equalto 0.0010% Note that, such more enhanced effect of toughness aftercarburizing will saturate if the content of B exceeds 0.01%, so that thecontent of B is preferably specified to be less than or equal to 0.01%.The content of B is more preferably less than or equal to 0.0075%, evenmore preferably less than or equal to 0.0050%, and yet more preferablyless than or equal to 0.0020%.

[W: Less than or Equal to 1.0%]

W (tungsten) is an element that acts to deoxidize molten steel toimprove soundness of the steel. Hence in the steel sheet for carburizingaccording to the embodiment, W may be contained as needed at a maximumcontent of 1.0%. The content of W is more preferably less than or equalto 0.5%.

[Ca: Less than or Equal to 0.01%]

Ca (calcium) is an element that acts to deoxidize molten steel toimprove soundness of the steel. Hence in the steel sheet for carburizingaccording to the embodiment, Ca may be contained as needed at a maximumcontent of 0.01%. The content of Ca is more preferably less than orequal to 0.005%.

[Balance: Fe and Impurities]

The balance of the component composition at the center of thicknessincludes Fe and impurities. For example, the impurities are exemplifiedby elements derived from the starting steel or scrap, and/orincorporated in the process of steel making, which are acceptable solong as characteristics of the steel sheet for carburizing according tothe embodiment will not be adversely affected.

Chemical components contained in the steel sheet for carburizingaccording to the embodiment have been detailed.

<Microstructure of Steel Sheet for Carburizing>

Next, the microstructure that makes up the steel sheet for carburizingaccording to the embodiment will be detailed.

The microstructure of the steel sheet for carburizing according to theembodiment is substantially composed of ferrite and carbide. In moredetail, the microstructure of the steel sheet for carburizing accordingto the embodiment is composed so that the average crystal grain size offerrite is smaller than 10 μm, the percentage of area of ferritetypically falls in the range from 80 to 95%, the percentage of area ofcarbide typically falls in the range from 5 to 20%, and the totalpercentage of area of ferrite and carbide will not exceed 100%.

Such percentages of area of ferrite and carbide are measured by using asample sampled from the steel sheet for carburizing so as to produce thecross section to be observed in the direction perpendicular to the widthdirection. A length of sample of 10 mm to 25 mm or around will suffice,although depending on types of measuring instrument. The surface to beobserved of the sample is polished, and then etched using nital. Thesurface to be observed, after etched with nital, is observed in regionsat a quarter thickness position (which means a position in the thicknessdirection of the steel sheet for carburizing, quarter thickness awayfrom the surface), at a ⅜ thickness position, and at the half thicknessposition, under a thermal-field-emission type scanning electronmicroscope (for example, JSM-7001F from JEOL, Ltd.).

Each sample is observed for the regions having an area of 2500 μm² inten fields of view, and percentages of areas occupied by ferrite andcarbide relative to the area of field of view are measured for eachfield of view. An average value of percentages of area occupied byferrite, being averaged from all fields of view, and, an average valueof percentages of area occupied by carbide, being averaged from allfields of view, are respectively denoted as the percentage of area offerrite, and, the percentage of area of carbide.

Now the carbide in the microstructure according to the embodiment ismainly iron carbide such as cementite which is a compound of iron andcarbon (Fe₃C), and, ε carbide (Fe₂₋₃C). Alternatively, besides theaforementioned iron carbide, the carbide in the microstructureoccasionally contains a compound derived from cementite having Fe atomssubstituted by Mn, Cr and so forth, and alloy carbides (such as M₂₃C₆,M₆C and MC, where M represents Fe and other metal element). Most part ofthe carbide in the microstructure according to the embodiment iscomposed of iron carbide. Hence, focusing now on the later-detailednumber of such carbides, the number may be the total number of theaforementioned various carbides, or may be the number of iron carbideonly. That is, the later-described various percentages of the number ofcarbides may be defined on the basis of a population that containsvarious carbides including iron carbide, or may be defined on the basisof a population that contains iron carbide only. The iron carbide may beidentified typically by subjecting the sample to diffractometry or EDS(Energy Dispersive X-ray spectrometry).

In bending deformation, deformation stress is concentrated at theinterface between a soft structure and a hard structure. It is thereforedesired to reduce as possible difference of hardness between the softstructure and hard structure, or, to control geometry of the hardstructure so as to relieve the stress concentration. Now the cracks maybe suppressed from generating by reducing the aspect ratio of carbidethrough spherodizing annealing. As the bending deformation furtherproceeds, the produced cracks may extend. Since the cracks propagatethrough regions where fracture is likely to occur, grain boundary offerrite, and, interface between ferrite and carbide may serve as routesfor propagation. In this process, since the carbide if produced in thegrain boundary of ferrite can assist extension of the cracks whilerouted through the grain boundary, the carbide is desired to be producedwithin crystal grains of ferrite. Propagation of cracks through thegrain boundary is considered to be suppressible, by producing thecarbide within the ferrite crystal grains.

The carburized member will have carbon introduced by carburizing in theskin layer, so that the member will have high strength in the skinlayer, whereas the steel material as a starting material for thecarburized member will become brittle as the strength increases. Hence,the toughness of the skin layer holds the key for the steel sheet forcarburizing as the starting material. Regarding this point, thetoughness is improved by micronizing crystal grains in the skin layer ofthe steel sheet. As will be detailed below, by annealing the steel sheetin a nitrogen-rich atmosphere, the atmospheric nitrogen enters the steelsheet to form nitride in the skin layer of the steel sheet. The thusformed nitride is mainly composed of fine AlN, and can demonstrate aneffect of suppressing growth of grains of prior austenite duringcarburizing heat treatment. Grain size of prior austenite and grain sizeof transformed martensite are in a proportional relationship. It wastherefore made clear that if the grains of prior austenite aresuppressed from growing by such fine AIN, also the grain size ofmartensite in a structure of carburized member can be micronized.

Reasons for limitations of the microstructure that composes the steelsheet for carburizing according to this embodiment will be detailedbelow.

[Average Crystal grain size of Ferrite: Smaller than 10 μm]

In the microstructure of the steel sheet for carburizing according tothis embodiment, the average crystal grain size of ferrite is specifiedto be smaller than 10 μm as described above. With the average crystalgrain size of ferrite specified to be smaller than 10 μm, theaforementioned effect through micronization of crystal grains may bedemonstrated, and the impact value after carburizing may be improved.With the average crystal grain size of ferrite set to 10 μm or larger,the aforementioned effect through micronization of crystal grains willnot be obtained, failing in improving the impact value aftercarburizing. The average crystal grain size of ferrite is preferablysmaller than 8 μm. The lower limit value of the average crystal grainsize of ferrite is not specifically limited. Since, however, it isdifficult to control the average crystal grain size of ferrite smallerthan 0.1 μm in practical operation, 0.1 μm is understood as asubstantial lower limit.

[Percentage of Number of Carbides with Aspect Ratio of 2.0 or Smaller,Relative to Total Carbides: 80% or Larger]

As described previously, the carbide according to the embodiment ismainly composed of iron carbides such as cementite (Fe₃C) and, c carbide(Fe₂₋₃C). Investigation by the present inventors revealed that goodbendability is obtainable, if the percentage of the number of carbideswith an aspect ratio of 2.0 or smaller, relative to the total carbides,is 80% or larger. With the percentage of the number of carbides with anaspect ratio of 2.0 or smaller relative to the total carbides fallenbelow 80%, good bendability will not be obtained due to acceleratedcracking during bending deformation. Therefore in the steel sheet forcarburizing according to the embodiment, the lower limit value of thepercentage of the number of carbides with an aspect ratio of 2.0 orsmaller, relative to the total carbides, is specified to be 80%. Thepercentage of the number of carbides with an aspect ratio of 2.0 orsmaller relative to the total carbides is more preferably 85% or larger,for further improvement of the bendability. Note that there is nospecial limitation on the upper limit of the percentage of the number ofcarbides with an aspect ratio of 2.0 or smaller relative to the totalcarbides. Since, however, it is difficult to achieve 98% or larger inpractical operation, 98% will be a substantial upper limit.

[Percentage of Number of Carbides Present in Ferrite crystal grain,Relative to Total Carbides: 60% or Larger]

Investigations by the present inventors revealed that good bendabilityis obtainable, if the percentage of the number of carbides present inferrite crystal grain, relative to total carbides, is 60% or larger.With the percentage of the number of carbides present in ferrite crystalgrain relative to total carbides fallen under 60%, good bendability willnot be obtained due to accelerated cracking during bending deformation.Therefore in the steel sheet for carburizing according to theembodiment, the lower limit value of the percentage of the number ofcarbides present in ferrite crystal grain, relative to total carbides,is specified to be 60%. The percentage of the number of carbides presentin ferrite crystal grain relative to total carbides is more preferably65% or larger, for further improvement of the bendability. Note thatthere is no special limitation on the upper limit of the percentage ofthe number of carbides present in ferrite crystal grain relative to thetotal carbides. Since, however, it is difficult to achieve 98% or largerin practical operation, 98% will be a substantial upper limit.

[Average Equivalent Circle Diameter of Carbide: 5.0 μm or Smaller]

In the microstructure of the steel sheet for carburizing according tothe embodiment, the average equivalent circle diameter of carbide needbe 5.0 μm or smaller. With the average equivalent circle diameter ofcarbide exceeding 5.0 μm, good bendability will not be obtained due tocracking that occurs during bending deformation. The smaller the averageequivalent circle diameter of carbide is, the better the bendability.The average equivalent circle diameter is preferably 1.0 μm or smaller,more preferably 0.8 μm or smaller, and even more preferably 0.6 μm orsmaller. The lower limit value of the average equivalent circle diameterof carbide is not specifically limited. Since, however, it is difficultto achieve an average equivalent circle diameter of carbide of 0.01 μmor smaller in practical operation, 0.01 μm will be a substantial lowerlimit.

Next, methods for measuring the average grain size of ferrite in themicrostructure, and, various percentages of the number of carbides andthe average equivalent circle diameter of carbide will be detailed. Notethat the measurement below employed fixed positions of observation ofsamples, but there is no large difference between the states of ferriteand carbide measured in the samples, and the states of ferrite andcarbide in the skin layer (nitrogen-rich region) of the steel sheetaccording to this embodiment.

First, a sample is cut out from the steel sheet for carburizing, so asto produce a cross section to be observed, which is perpendicular to thesurface (thickness-wise cross section). A length of sample of 10 mm oraround will suffice, although depending on types of measuringinstrument. The cross section is polished and corroded, and is thensubjected to measurement of position of precipitation, aspect ratio, andaverage equivalent circle diameter of carbide. For the polishing, itsuffices for example to polish the surface to be measured using a600-grit to 1500-grit silicon carbide sandpaper, and then to specularlyfinish the surface using a liquid having diamond powder of 1 μm to 6 μmin diameter dispersed in a diluent such as alcohol or in water. Thecorrosion is not specifically limited so long as the shape and positionof precipitation of carbide can be observed. In order to corrode thegrain boundary between carbide and matrix iron, it is suitable toemploy, for example, etching using a saturated picric acid-alcoholsolution; or a method for removing the matrix iron to a depth of severalmicrometers typically by potentiostatic electrolytic etching using anonaqueous solvent-based electrolyte (Fumio Kurosawa et al., Journal ofthe Japan Institute of Metals and Materials (in Japanese), 43, 1068,(1979)), so as to allow the carbide only to remain.

The average crystal grain size of ferrite is estimated by photographinga 2500 μm² area at around a quarter thickness position of the sampleunder a thermal-field-emission type scanning electron microscope (forexample, JSM-7001F from JEOL, Ltd.), and by applying the line segmentmethod to the captured images.

The aspect ratio of carbide is estimated by observing a 10000 μm² areaat around a quarter thickness position of the sample, under athermal-field-emission type scanning electron microscope (for example,JSM-7001F from JEOL, Ltd.). All carbides contained in an observed fieldof view are measured regarding the long axes and the short axes tocalculate aspect ratios (long axis/short axis), and an average value ofthe aspect ratios is determined. Such observation is made in five fieldsof view, and an average value for these five fields of view isdetermined as the aspect ratio of carbide in the sample. Referring tothe thus obtained aspect ratio of carbide, the percentage of the numberof carbides with an aspect ratio of 2.0 or smaller relative to the totalcarbides is calculated, on the basis of the total number of carbideswith an aspect ratio of 2.0 or smaller, and the total number of carbidespresent in the five fields of view.

The position of precipitation of carbide is confirmed by observing a10000 μm² area at around a quarter thickness position of the sample,under a thermal-field-emission type scanning electron microscope (forexample, JSM-7001F from JEOL, Ltd.). All carbides contained in anobserved field of view are measured regarding the position ofprecipitation, and percentage of carbides that precipitated within theferrite crystal grain, relative to the total number of carbides, iscalculated. The observation is made in five fields of view, and anaverage value for these five fields of view is determined as thepercentage of carbides formed within the ferrite crystal grain, amongfrom the carbides (that is, the percentage of the number of carbidespresent within the ferrite crystal grain, among from the totalcarbides).

The average equivalent circle diameter of carbide is estimated byobserving a 600 μm² area at around a quarter thickness position of thesample in four fields of view, under a thermal-field-emission typescanning electron microscope (for example, JSM-7001F from JEOL, Ltd.).For each field of view, the long axes and the short axes of capturedcarbides are individually measured, using image analysis software (forexample, IMage-Pro Plus from Media Cybernetics, Inc.). For each carbidein the field of view, the long axis and the short axis are averaged toobtain the diameter of carbide, and the diameters obtained from allcarbides captured in the field of view are averaged. The thus obtainedaverage values of the diameter of carbides from four fields of view arefurther averaged by the number of fields of view, to determine theaverage equivalent circle diameter of carbide.

The microstructure possessed by the steel sheet for carburizingaccording to the embodiment has been detailed.

[Average Nitrogen Concentration of Skin Layer of Steel Sheet: 0.040 mass% or Higher, and 0.20 mass % or Smaller]

Next, the average nitrogen concentration in the skin layer of the steelsheet for carburizing will be explained. Investigations by the presentinventors revealed that, with the average nitrogen concentration in theskin layer of the steel sheet for carburizing controlled to 0.040 mass %or higher, the carburized members made of the steel sheet forcarburizing were successful in obtaining good toughness. Such findingswill be detailed below.

The present inventors sampled a thin film sample of 40 μm long and 25 μmdeep from a region around the skin layer of a carburized member whichwas found to show good toughness, using a focused ion beamprocessing/observation apparatus, and observed the microstructure undera transmission electron microscope. As a consequence, fine AlN with anaverage diameter of 50 nm or smaller was found to be produced in thethin film sample.

The present inventors further made an analysis as described below toinvestigate correlation between the position of production of AlN and amatrix structure. That is, a thin film sample of 100 μm long and 100 μmdeep, sampled using the focused ion beam processing/observationapparatus was fixed on a mesh holder made of copper, and analyzed usinga transmission electron backscatter diffractometer equipped on athermal-field-emission type scanning electron microscope (JSM-6500F,from JEOL, Ltd.). A crystal orientation map of prior austenite wasreorganized referring to the measurement results obtained from theelectron backscatter diffractometry, and compared with an image obtainedunder the transmission electron microscope. It was consequently madeclear that the fine MN resides at around grain boundary of prioraustenite, and that the grain boundary of prior austenite in which thefine AlN precipitated was found to reside over a range from the topmostsurface of steel sheet to a depth of 50 μm or around. More specifically,it was contemplated that the fine AlN, produced in the skin layer of thesteel sheet (a region ranging from the topmost surface of steel sheet toa depth of 50 μm) suppressed the prior austenite grains from growingduring carburization heat treatment, so that the grain size ofmartensite was micronized in the structure of carburized member, and theimpact value dramatically increased. Note that the topmost surface ofthe steel sheet in this context means the surface of the base materialof steel sheet, while excluding various layers including a scale layerwhich possibly resides on the surface of the base material of steelsheet.

The present inventors further analyzed the carburized member whosetoughness was found to be good, regarding a profile of nitrogenconcentration over a range from the surface of steel sheet up to thecenter of steel sheet, using an electron probe microanalyzer equippedwith a wavelength dispersive X-ray spectrometer and a field-emissionelectron gun. As a consequence, the skin layer of the steel sheet (thatis, the region ranging from the topmost surface of steel sheet to adepth of 50 μm) was confirmed to have an average nitrogen concentrationof 0.040 mass % or higher.

The present inventors confirmed after thorough investigations that theskin layer of the steel sheet will have an average nitrogenconcentration of 0.040 mass % or higher and 0.200 mass % or lower, byusing as a material the steel sheet with the average nitrogenconcentration in the middle-thickness portion (in more detail, theaverage nitrogen concentration over a range from the middle-thicknessportion up to 100 μm away towards the surface) controlled to 0.2 mass %or lower, by heating the steel sheet used as the material in anatmosphere with the nitrogen concentration controlled to 25% or higherin volume fraction, at an average heating rate of 5° C./h or higher and100° C./h or lower, up into a temperature range not higher than pointAc₁; by keeping the steel sheet in the temperature range not higher thanthe point Ac₁ for 10 h or longer and 100 h or shorter; and then bycooling the steel sheet at an average cooling rate of 5° C./h or higherand 100° C./h or lower. That is, by heating the steel sheet in anatmosphere with the nitrogen concentration controlled to 25% or higherin volume fraction, at an average heating rate of 5° C./h or higher and100° C./h or lower, up into a temperature range not higher than pointAc_(i); by keeping the steel sheet in the temperature range not higherthan the point Ac_(i) for 10 h or longer and 100 h or shorter, and thenby cooling the steel sheet at an average cooling rate of 5° C./h orhigher and 100° C./h or lower, the skin layer of the steel sheet willhave produced therein fine AlN of 50 nm or smaller. As a consequence,the skin layer of the steel sheet is understood to have an averagenitrogen concentration of 0.040 mass % or higher and 0.200 mass % orlower. Note that the aforementioned structure of fine AlN produced byannealing will remain almost unmodified throughout cold-working, andwill contribute to suppress the prior austenite grains from growingduring carburization heat treatment.

As described above, the thorough investigations by the present inventorsrevealed that, with the average nitrogen concentration controlled to0.040 mass % or higher in the skin layer of steel sheet (the regionranging from the topmost surface of steel sheet to a depth of 50 μm) ofthe steel sheet for carburizing, the skin layer of the steel sheet willhave produced therein fine AIN, and the impact value will be improved inthe carburized member. The average nitrogen concentration in the skinlayer of the steel sheet is preferably 0.045 mass % or higher.Meanwhile, with the average nitrogen concentration exceeding 0.200 mass% in the skin layer of the steel sheet, coarse nitride will be producedto degrade the toughness. The average nitrogen concentration in the skinlayer of the steel sheet is therefore specified to be 0.200 mass % atmaximum. The average nitrogen concentration in the skin layer of thesteel sheet is preferably 0.150 mass % or lower.

Next, a method for determining the average nitrogen concentration on thesurface of steel sheet will be explained.

As mentioned previously, the structure of fine AIN produced by annealingwill remain almost unchanged throughout cold-working, and contributes tosuppress the prior austenite grains from growing during carburizationheat treatment. Hence, it suffices to examine the nitrogen profile,using the steel sheet for carburizing obtained after annealing ahot-rolled steel sheet or a cold-rolled steel sheet.

More specifically, a sample is cut out from the steel sheet forcarburizing, so as to produce a cross section to be observed, which isperpendicular to the surface (thickness-wise cross section). A length ofsample of 10 mm to 25 mm or around will suffice, although depending ontypes of measuring instrument. The surface to be measured is preparedunder argon ion beam so as not to produce streak-like irregularity overthe surface to be measured, using a cross section polisher from JEOL,Ltd. and a sample rotating holder from JEOL, Ltd. Thereafter by using anelectron probe microanalyzer equipped with a wavelength dispersive X-rayspectrometer and an field-emission electron gun, the nitrogenconcentration profile is measured over a range from the topmost surfaceof the steel sheet up to the middle-thickness portion (half-thicknessposition) at 50 nm intervals. An average value of the nitrogenconcentration (in mass %) over a range from the topmost surface of thesteel sheet up to a 50 μm deep point is then calculated, and isspecified to be aforementioned average nitrogen concentration of theskin layer of the steel sheet. In addition, the average value of thenitrogen concentration (in mass %) over a range from themiddle-thickness portion up to 100 μm away towards the surface isspecified to be the average nitrogen concentration in themiddle-thickness portion. Note that the amount of intrusion of nitrogenin the annealing step does not largely differ between the top and backsurfaces of a coil, so that the measurement made only either on the topor back surface of the steel sheet will suffice.

<Thickness of Steel Sheet for Carburizing>

The thickness of the steel sheet for carburizing according to theembodiment is not specifically limited, but is preferably 2 mm orlarger, for example. With the thickness of the steel sheet forcarburizing specified to be 2 mm or larger, difference of thickness inthe coil width direction may further be reduced. The thickness of thesteel sheet for carburizing is more preferably 2.3 mm or larger.Further, the thickness of the steel sheet for carburizing is notspecifically limited, but is preferably 6 mm or smaller. With thethickness of the steel sheet for carburizing specified to be 6 mm orsmaller, load of press forming may be reduced, making forming intocomponents easier. The thickness of the steel sheet for carburizing ismore preferably 5.8 mm or smaller.

The steel sheet for carburizing according to the embodiment has beendetailed.

(Method for Manufacturing Steel Sheet for Carburizing)

Next, a method for manufacturing the above-explained steel sheet forcarburizing according to the embodiment will be detailed.

The method for manufacturing the above-explained steel sheet forcarburizing according to the embodiment includes (A) a hot-rolling stepin which a steel material having the chemical composition explainedabove is used to manufacture the hot-rolled steel sheet according topredetermined conditions, and (B) an annealing step in which the thusobtained hot-rolled steel sheet, or the steel sheet having beencold-rolled subsequently to the hot-rolling step is annealed accordingto predetermined heat treatment conditions.

The hot-rolling step and the annealing step will be detailed below.

<Hot-Rolling Step>

The hot-rolling step described below is a step in which a steel materialhaving the predetermined chemical composition is used to manufacture thehot-rolled steel sheet according to the predetermined conditions.

Steel billet (steel material) subjected now to hot-rolling may be anybillet manufactured by any of usual methods. For example, employable isa billet manufactured by any of usual methods, such as continuously castslab and thin slab caster.

In addition, from the viewpoint of improving the toughness and regardingthe inclusions such as MnS or center segregation of Mn in the steelmaterial to be hot-rolled, the fewer the better. Hence, for example, inthe continuous casting step for obtaining a billet to be hot-rolled, itis preferable to carry out soundness enhancing treatment of the steelmaterial, such as producing a predetermined inclusion by controlling theamount of pouring of molten steel per unit time, or such as reducing thecenter segregation before the billet completely solidifies.

In more detail, using the steel material having the aforementionedchemical composition, the steel material is heated and subjected tohot-rolling, the hot finish rolling is terminated in the temperaturerange of 800° C. or higher and lower than 920° C., and then wound up ata temperature of 700° C. or lower, to thereby manufacture the hot-rolledsteel sheet. In this process, the cooling after the hot finish rollingis started within one second after the end of the hot finish rolling,and the average cooling rate after the hot finish rolling is specifiedto be higher than 50° C./s.

[Rolling Temperature in Hot Finish Rolling: 800° C. or Higher, and Lowerthan 920° C.]

In the hot-rolling step according to this embodiment, rolling in the hotfinish rolling need take place at a rolling temperature of 800° C. orhigher. With the rolling temperature during the hot finish rolling (thatis, the finish rolling temperature) dropped below 800° C., alsotemperature at which ferrite transformation starts will drop, to therebycoarsen the carbides to be precipitated. As a consequence, these coarsecarbides are acceleratingly grown in the annealing step in thesucceeding stage, to degrade the bendability. The finish rollingtemperature in the hot-rolling step according to this embodiment istherefore specified to be 800° C. or higher. The finish rollingtemperature is preferably 830° C. or higher. Meanwhile, with the finishrolling temperature reached 920° C. or higher, austenite grains will bedistinctively coarsened to reduce sites of nucleation of ferrite, thetemperature at which ferrite transformation starts will thus be lowered,making the carbides to be precipitated more easily be coarsened. In thiscase, theses coarse carbides are acceleratingly grown in the annealingstep in the succeeding stage, to degrade the bendability. The finishrolling temperature in the hot-rolling step according to this embodimentis therefore specified to be lower than 920° C. The finish rollingtemperature is preferably lower than 900° C.

[Winding Temperature: 700° C. or Lower]

As mentioned previously, the microstructure of the steel sheet forcarburizing need be featured by that the percentage of the number ofcarbides with an aspect ratio of 2.0 or smaller relative to the totalcarbides is 80% or larger; that the percentage of the number of carbidespresent in ferrite crystal grain relative to the total carbides is 60%or larger; that the average equivalent circle diameter of carbide is 5.0μm or smaller; and that the average nitrogen concentration in the skinlayer of the steel sheet is 0.040 mass % or higher and 0.200 mass % orlower. Accordingly, the steel sheet before being subjected to theannealing step in the succeeding stage (in more detail, spherodizingannealing) preferably has a structure (hot-rolled steel sheet structure)that mainly includes 10% or more and 80% or less, in percentage of area,of ferrite, and 10% or more and 60% or less, in percentage of area, ofpearlite, totaling 100% or less in percentage of area, and the balancethat preferably includes at least any of bainite, martensite, temperedmartensite and residual austenite.

If the winding temperature in the hot-rolling step according to theembodiment exceeds 700° C., ferrite transformation will be excessivelypromoted to suppress production of pearlite, making it difficult tocontrol, in the steel sheet for carburizing after the annealing, thepercentage of number of carbides with an aspect ratio of 2.0 or smaller,among from the total carbides, to 80% or larger. Hence in thehot-rolling step according to the embodiment, the upper limit of thewinding temperature is specified to be 700° C. The lower limit of thewinding temperature in the hot-rolling step according to the embodimentis not specifically limited. Since, however, winding at room temperatureor below is difficult in practical operation, room temperature will be asubstantial lower limit. Note that the winding temperature in thehot-rolling step according to the embodiment is preferably 400° C. orhigher, from the viewpoint of further reducing the aspect ratio ofcarbide in the annealing step in the succeeding stage.

[Cooling Start Time after Hot Finish Rolling: within One Minute afterEnd of Hot Finish Rolling][Average Cooling Rate after Hot Finish Rolling: Higher than 50° C./s]

In the hot-rolling step according to this embodiment, cooling at anaverage cooling rate of higher than 50° C./s is started within onesecond after the end of the hot finish rolling. In this way, austenitegrains after the hot finish rolling may be micronized. With theaustenite grains micronized after the hot finish rolling, it now becomespossible to control the average grain size of ferrite, after theannealing step (in more detail, spherodizing annealing) in thesucceeding stage to smaller than 10 μm.

With the cooling start time fallen behind one second after the end ofhot finish rolling, the austenite grains will be coarsened, so that theaverage crystal grain size of ferrite after spherodizing annealing willexceed 10 μm, making it unable to exhibit the effect of micronizing thecrystal grains. The cooling start time after the hot finish rollingpreferably falls within 0.8 seconds after the end. The lower limit ofthe cooling start time is not specifically limited. Note however that itis difficult to allow the cooling start time to fall within 0.01 secondsafter the end in practical operation, so that 0.01 seconds is understoodas a substantial lower limit.

Meanwhile, with the average cooling rate after the hot finish rollingfallen to 50° C./s or lower, the austenite grains will be coarsened, sothat the average crystal grain size of ferrite after spherodizingannealing in the succeeding stage will exceed 10 μm. The average coolingrate after the hot finish rolling is preferably 55° C./s or higher. Theupper limit of the average cooling rate is not specifically limited.Note however that it is difficult to control the average cooling rate to300° C./s or higher in practical operation, so that 300° C./s isunderstood as a substantial upper limit.

Alternatively, the steel sheet thus wound up in the aforementionedhot-rolling step (hot-rolled steel sheet) may be unwound, pickled, andthen cold-rolled. Through removal of oxide on the surface of steel sheetby pickling, the hole expandability may further be improved. Thepickling may be carried out once, or may be carried out in multipletimes. The cold-rolling may be carried out at an ordinary draft (30 to90%, for example). The hot-rolled steel sheet and cold-rolled steelsheet also include steel sheet temper-rolled under usual conditions,besides the steel sheets that are left unmodified after hot-rolled orcold-rolled.

The hot-rolled steel sheet is manufactured as described above, in thehot-rolling step according to the embodiment. The thus manufacturedhot-rolled steel sheet, or, the steel sheet having been cold-rolledsubsequently to the hot-rolling step may further be subjected tospecific annealing in the annealing step detailed below, to obtain thesteel sheet for carburizing according to the embodiment.

<Annealing Step>

The annealing step detailed below is a step in which the hot-rolledsteel sheet obtained in the aforementioned hot-rolling step, or, thesteel sheet having been cold-rolled subsequently to the hot-rolling stepis subjected to annealing (spherodizing annealing) under predeterminedheat treatment conditions. Through the annealing, pearlite having beenproduced in the hot-rolling step is spherodized, and the average crystalgrain size of ferrite after spherodizing annealing is controlled tosmaller than 10 μm.

In more detail, the hot-rolled steel sheet obtained as described above,or, the steel sheet having been cold-rolled subsequently to thehot-rolling step is heated in an atmosphere with nitrogen concentrationcontrolled to 25% or higher in volume fraction, at an average heatingrate of 5° C./h or higher and 100° C./h or lower, up into a temperaturerange not higher than point Ac_(i) defined by equation (101) below,annealed in a temperature range not higher than the point Ac_(i) for 10h or longer and 100 h or shorter, and then cooled at an average coolingrate of 5° C./h or higher and 100° C./h or lower in a temperature rangefrom a temperature at the end of annealing down to 550° C.

Now in the equation (101) below, the notation [X] represents the contentof element X (in mass %), which will be substituted by zero if suchelement X is absent.

$\begin{matrix}\left\lbrack {{Math}.\mspace{14mu} 2} \right\rbrack & \; \\{{Ac}_{1} = {750.8 - {26.6\lbrack C\rbrack} + {17.6\lbrack{Si}\rbrack} - {11.6\lbrack{Mn}\rbrack} - {22.9\lbrack{Cu}\rbrack} - {23\lbrack{Ni}\rbrack} + {24.1\lbrack{Cr}\rbrack} + {22.5\lbrack{Mo}\rbrack} - {39.7\lbrack V\rbrack} - {5.7\lbrack{Ti}\rbrack} + {232.4\lbrack{Nb}\rbrack} - {169.4\lbrack{Al}\rbrack} - {894.7\lbrack B\rbrack}}} & {{Equation}\mspace{14mu} (101)}\end{matrix}$

[Annealing Atmosphere: Atmosphere with Nitrogen Concentration Controlledto 25% or Higher in Volume Fraction]

In the aforementioned annealing step, the annealing atmosphere will becontrolled to have a nitrogen concentration of 25% or higher in volumefraction. With the nitrogen concentration fallen below 25% in volumefraction, the average nitrogen concentration in the skin layer of thesteel sheet will no longer be controlled to 0.040 mass % or higher and0.200 mass % or lower. Hence, in the annealing step according to thisembodiment, the nitrogen concentration in the annealing atmosphere isspecified to be 25% or higher in volume fraction. The nitrogenconcentration in the annealing atmosphere is preferably 75% or higher involume fraction, and even more preferably 80% or higher in volumefraction. Note that the higher the nitrogen concentration, the better.Since it is, however, not cost-effective to control the nitrogenconcentration to 99% or higher in volume fraction, 99% in volumefraction is understood as a substantial upper limit.

In the annealing step according to this embodiment, the heat treatmentis carried out while introducing, as the atmospheric gas, a gas that iscomposed of a molecule containing nitrogen atom, while controlling theannealing atmosphere. For example, it suffices to control the annealingatmosphere typically by regulating flow rate of the atmospheric gas tobe introduced into a heating furnace used for the annealing step, usinga gas concentration gauge installed in an annealing furnace.

Note that the balance of the atmospheric gas may be mainly composed ofany inert gas other than nitrogen, allowing appropriate use of reducinggas such as hydrogen and argon, for example. More specifically, theannealing atmosphere may have a nitrogen concentration of 75% or higherin volume fraction, with the balance of hydrogen. Alternatively, theatmospheric gas may contain a gas such as oxygen if the content is notso large.

[Heating Condition: at Average Heating Rate of 5° C./h or Higher and100° C./h or Lower, up into Temperature Range not Higher than Point Ac₁]

In the annealing step according to the embodiment, the aforementionedhot-rolled steel sheet, or, the steel sheet having been cold-rolledsubsequently to the hot-rolling step need be heated at an averageheating rate of 5° C./h or higher and 100° C./h or lower, up into atemperature range not higher than point Ac₁ defined by the equation(101) above. With the average heating rate set lower than 5° C./h, theaverage equivalent circle diameter of carbide will exceed 5.0 μm,degrading the bendability. Meanwhile, with an average heating rateexceeding 100° C./h, spherodizing of carbide will not be fully promoted,making it difficult to control the percentage of the number of carbideswith an aspect ratio of 2.0 or smaller, among from the total carbides,to 80% or larger. Further, at a heating temperature exceeding point Ac₁defined by the equation (101) above, the percentage of the number ofcarbides formed within the ferrite crystal grains among from the totalcarbides will fall under 60%, making it unsuccessful to obtain goodbendability. Note that the lower limit of the temperature range ofheating temperature is not specifically limited. However, in thetemperature range of heating temperature below 600° C., retention timein annealing process will become longer, making the process notcost-effective. Hence, the temperature range of heating temperature ispreferably specified to be 600° C. or higher. For more proper control ofthe state of carbide, the average heating rate in the annealing stepaccording to the embodiment is preferably specified to be 20° C./h orhigher. Further, for more proper control of the state of carbide, theaverage heating temperature in the annealing step according to theembodiment is preferably specified to be 50° C./h or lower. For moreproper control of the state of carbide, the temperature range of heatingtemperature in the annealing step according to the embodiment is morepreferably specified to be 630° C. or higher. Furthermore, for moreproper control of the state of carbide, the temperature range of heatingtemperature in the annealing step according to the embodiment is morepreferably specified to be 670° C. or lower.

[Retention Time: in Temperature Range not Higher than Point Ac₁, for 10h or Longer and 100 h or Shorter]

In the annealing step according to the embodiment, the aforementionedtemperature range not higher than point Ac₁ (preferably, 600° C. orhigher and point Ac₁ or lower) need be kept for 10 h or longer and 100 hor shorter. With the retention time set shorter than 10 h, spherodizingof carbide will not be fully promoted, making it difficult to controlthe percentage of the number of carbides with an aspect ratio of 2.0 orsmaller, among from the total carbides, to 80% or larger. Meanwhile,with the retention time exceeding 100 h, the average equivalent circlediameter of carbide will exceed 5.0 μm, degrading the bendability. Formore proper control of the state of carbide, the retention time in theannealing step according to the embodiment is preferably 20 h or longer.Further, for more proper control of the state of carbide, the retentiontime in the annealing step according to the embodiment is preferably 80h or shorter.

[Cooling Conditions: Cooled at Average Cooling Rate of 5° C./h or Higherand 100° C./h or Lower]

In the annealing step according to the embodiment, the steel sheet afterthe aforementioned retention under heating, is cooled at an averagecooling rate of 5° C./h or higher and 100° C./h or lower. Now theaverage cooling rate in this context means an average cooling rate overthe range from the temperature of retention under heating (in otherwords, the temperature at the end of annealing) down to 550° C. With theaverage cooling rate set below 5° C./h, the carbide will be excessivelycoarsened, degrading the bendability. Meanwhile, with the averagecooling rate exceeding 100° C./h, spherodizing of carbide will not befully promoted, making it difficult to control the percentage of thenumber of carbides with an aspect ratio of 2.0 or smaller, among fromthe total carbides, to 80% or larger. For more proper control of thestate of carbide, the average cooling rate over the range from thetemperature of retention under heating down to 550° C. is preferablyspecified to be 20° C./h or higher. Further, for more proper control ofthe state of carbide, the average cooling rate over the range from thetemperature of retention under heating down to 550° C. in the annealingstep according to the embodiment is preferably specified to be 50° C./hor lower.

Note that, in the annealing step according to the embodiment, theaverage cooling rate in a temperature range below 550° C. is notspecifically limited, allowing cooling at a freely selectable averagecooling rate down into a predetermined temperature range. The lowerlimit of temperature at which the cooling is terminated is notspecifically limited. Since, however, cooling below room temperature isdifficult in practical operation, room temperature will be a substantiallower limit.

The annealing step according to the embodiment has been detailed.

By carrying out the aforementioned hot-rolling step and annealing step,the above-explained steel sheet for carburizing according to theembodiment may be manufactured.

Note that, prior to the above-explained annealing step, the hot-rolledsteel sheet may be retained in the atmospheric air within thetemperature range of 40° C. or higher and 70° C. or lower, for 72 h orlonger and 350 h or shorter. Through such retention, it now becomespossible to form an aggregate of carbon solid-soluted in the ferritecrystal grain. The aggregate of carbon is an article formed by severalcarbon atoms aggregated in the ferrite crystal grain. Formation of suchaggregate of carbon can further promote formation of carbide in theannealing step in the succeeding stage. As a consequence, mobility ofdislocation in the annealed steel sheet may further be improved, andthereby formability of the annealed steel sheet may further be improved.

Moreover, the thus obtained steel sheet for carburizing may be, forexample, subjected to cold working as a post-process. Further, the thuscold-worked steel sheet for carburizing may be subjected tocarburization heat treatment, typically within a carbon potential rangeof 0.4 to 1.0 mass %. Conditions for the carburization heat treatmentare not specifically limited, and may be appropriately controlled so asto obtain desired characteristics. For example, the steel sheet forcarburizing may be heated up to a temperature that corresponds to theaustenitic single phase, carburized, and then cooled naturally down toroom temperature; or may be cooled once down to room temperature,reheated, and then quickly quenched. Furthermore, for the purpose ofcontrolling the strength, the entire portion or part of the member maybe tempered. Alternatively, the steel sheet may be plated on the surfacefor the purpose of obtaining a rust-proofing effect, or may be subjectedto shot peening on the surface for the purpose of improving fatiguecharacteristics.

EXAMPLES

Next, examples of the present invention will be explained. Note thatconditions described in examples are merely exemplary conditionsemployed in order to confirm feasibility and effects of the presentinvention. The present invention is not limited to these exemplaryconditions. The present invention can employ various conditions withoutdeparting from the spirit of the present invention, insofar as thepurpose of the present invention will be achieved.

TEST EXAMPLES

Steel materials having chemical compositions listed in Table 1 belowwere hot-rolled (and cold-rolled) according to conditions listed inTable 2, and then annealed, to obtain the steel sheets for carburizing.The hot-rolling according to the conditions listed in Table 2 below wasfollowed by retention in the atmospheric air at 55° C. for 105 hours,and by annealing according to conditions listed in Table 2. Now inexemplary conditions listed in Table 2 below, in the continuous castingstep for obtaining the steel material to be subjected to hot-rolling,the soundness enhancing treatment of the steel material was carried outby controlling the amount of pouring of molten steel per unit time. Notethat in Table 1 and Table 2, the underlines are used to indicatedeviation from the scope of invention.

TABLE 1-1 Chemical Ingredients of Matrix Steel Sheet (in mass %, Balanceis Fe and Impurities.) No. C Si Mn P S sol. Al N Cr Mo Ni Cu Co Nb Ti 10.03 0.020 0.21 0.018 0.0040 0.0180 0.0101 0.030 0.836 0.000 0.000 0.0000.000 0.008 2 0.08 0.010 0.38 0.014 0.0051 0.0230 0.0064 0.020 0.8550.000 0.000 0.000 0.000 0.009 3 0.14 0.010 0.66 0.014 0.0038 0.05300.0051 0.020 0.018 0.000 0.000 0.000 0.000 0.004 4 0.05 0.110 1.54 0.0140.0041 0.0190 0.0116 0.250 0.654 0.000 0.000 0.000 0.000 0.005 5 0.080.100 2.10 0.017 0.0051 0.0260 0.0050 0.000 0.000 0.000 0.000 0.0000.000 0.000 6 0.13 0.030 0.76 0.013 0.0052 0.0110 0.0057 1.510 0.0170.000 0.000 0.000 0.000 0.004 7 0.01 0.010 0.50 0.013 0.0038 0.03700.0088 0.000 0.000 0.000 0.000 0.000 0.000 0.000 8 0.18 0.020 0.54 0.0160.0049 0.0340 0.0035 0.000 0.000 0.000 0.000 0.000 0.000 0.000 9 0.220.030 0.44 0.018 0.0045 0.0320 0.0111 0.000 0.000 0.000 0.000 0.0000.000 0.000 10 0.27 0.010 0.39 0.018 0.0054 0.0460 0.0116 0.000 0.0000.000 0.000 0.000 0.000 0.000 11 0.41 0.030 0.49 0.018 0.0053 0.01500.0089 0.000 0.000 0.000 0.000 0.000 0.000 0.000 12 0.07 0.001 0.450.015 0.0055 0.0220 0.0097 0.000 0.000 0.000 0.000 0.000 0.000 0.000 130.08 1.540 0.53 0.017 0.0038 0.0140 0.0113 0.000 0.000 0.000 0.000 0.0000.000 0.000 14 0.06 0.020  0.002 0.014 0.0050 0.0440 0.0097 0.000 0.0000.000 0.000 0.000 0.000 0.000 15 0.08 0.020 3.55 0.016 0.0046 0.01500.0115 0.000 0.000 0.000 0.000 0.000 0.000 0.000 16 0.06 0.020 0.480.015 0.0037 0.0320 0.0062 1.520 0.000 0.000 0.000 0.000 0.000 0.000 170.07 0.010 0.51 0.016 0.0050 0.0310 0.0071 0.000 0.540 0.000 0.000 0.0000.000 0.000 18 0.08 0.030 0.54 0.015 0.0047 0.0290 0.0033 0.000 0.0000.390 0.000 0.000 0.000 0.000 19 0.08 0.030 0.54 0.016 0.0039 0.01300.0104 0.000 0.000 0.000 0.700 0.000 0.000 0.000 20 0.08 0.030 0.430.017 0.0036 0.0320 0.0066 0.000 0.000 0.000 0.000 0.550 0.000 0.000 210.06 0.020 0.39 0.017 0.0045 0.0420 0.0078 0.000 0.000 0.000 0.000 0.0000.032 0.000 22 0.07 0.010 0.49 0.015 0.0046 0.0370 0.0098 0.000 0.0000.000 0.000 0.000 0.000 0.018 23 0.06 0.020 0.58 0.014 0.0051 0.04900.0063 0.000 0.000 0.000 0.000 0.000 0.000 0.000 24 0.07 0.010 0.540.014 0.0046 0.0350 0.0043 0.000 0.000 0.000 0.000 0.000 0.000 0.000 250.08 0.010 0.48 0.013 0.0050 0.0360 0.0043 0.000 0.000 0.000 0.000 0.0000.000 0.000 26 0.07 0.020 0.58 0.015 0.0038 0.0140 0.0056 0.000 0.0000.000 0.000 0.000 0.000 0.000 27 0.07 0.020 0.50 0.018 0.0038 0.04600.0065 0.000 0.000 0.000 0.000 0.000 0.000 0.000 28 0.06 0.030 0.470.017 0.0048 0.0400 0.0119 0.000 0.000 0.000 0.000 0.000 0.000 0.000 290.08 0.006 0.38 0.016 0.0056 0.0153 0.0048 0.000 0.000 0.000 0.000 0.0000.000 0.008 30 0.07 0.460 0.40 0.018 0.0058 0.0150 0.0043 0.000 0.0000.000 0.000 0.000 0.000 0.000 Chemical Ingredients of Matrix Steel Sheet(in mass %, Balance is Fe and Impurities.) Ac₁ No. V B W Ca (° C.)Remark 1 0.0000 0.0003 0.00 0.000 764 2 0.0000 0.0004 0.00 0.000 760 30.0000 0.0001 0.00 0.000 731 4 0.0000 0.0002 0.00 0.000 751 5 0.00000.0000 0.00 0.000 722 6 0.0000 0.0002 0.00 0.000 774 7 0.0000 0.00000.00 0.000 739 Comparative steel 8 0.0000 0.0000 0.00 0.000 734 9 0.00000.0000 0.00 0.000 735 10 0.0000 0.0000 0.00 0.000 731 11 0.0000 0.00000.00 0.000 732 Comparative steel 12 0.0000 0.0000 0.00 0.000 740Comparative steel 13 0.0000 0.0000 0.00 0.000 767 Comparative steel 140.0000 0.0000 0.00 0.000 742 Comparative steel 15 0.0000 0.0000 0.000.000 705 Comparative steel 16 0.0000 0.0000 0.00 0.000 775 17 0.00000.0000 0.00 0.000 750 18 0.0000 0.0000 0.00 0.000 729 19 0.0000 0.00000.00 0.000 725 20 0.0000 0.0000 0.00 0.000 739 21 0.0000 0.0000 0.000.000 745 22 0.0000 0.0000 0.00 0.000 737 23 0.0610 0.0000 0.00 0.000732 24 0.0000 0.0015 0.00 0.000 736 25 0.0000 0.0000 0.00 0.000 737 260.0000 0.0000 0.22 0.000 740 27 0.0000 0.0000 0.00 0.004 736 28 0.00000.0000 0.00 0.000 738 29 0.0000 0.0003 0.000 0.000 741 30 0.0000 0.00030.000 0.000 749

TABLE 1-2 Chemical Ingredients of Matrix Steel Sheet (in mass %, Balanceis Fe and Impurities.) No. C Si Mn P S sol. Al N Cr Mo Ni Cu Co Nb Ti 310.05 0.012 0.02 0.018 0.0056 0.0146 0.0043 0.000 0.000 0.000 0.000 0.0000.000 0.008 32 0.09 0.008 2.88 0.016 0.0052 0.0152 0.0046 0.000 0.0000.000 0.000 0.000 0.000 0.008 33 0.06 0.012 0.39 0.089 0.0053 0.01470.0049 0.000 0.000 0.000 0.000 0.000 0.000 0.008 34 0.08 0.009 0.370.019 0.0920 0.0154 0.0043 0.000 0.000 0.000 0.000 0.000 0.000 0.008 350.09 0.009 0.42 0.019 0.0051 0.0004 0.0046 0.000 0.000 0.000 0.000 0.0000.000 0.008 36 0.05 0.012 0.41 0.019 0.0059 2.9000 0.0048 0.000 0.0000.000 0.000 0.000 0.000 0.008 37 0.08 0.010 0.38 0.014 0.0051 0.02300.0002 0.020 0.855 0.000 0.000 0.000 0.000 0.009 38 0.08 0.010 0.380.014 0.0051 0.0230 0.0310 0.020 0.855 0.000 0.000 0.000 0.000 0.009 390.05 0.010 0.42 0.016 0.0052 0.0153 0.0044 0.007 0.000 0.000 0.000 0.0000.000 0.008 40 0.09 0.010 0.42 0.019 0.0057 0.0151 0.0049 2.940 0.0000.000 0.000 0.000 0.000 0.008 41 0.08 0.010 0.36 0.018 0.0051 0.01500.0043 0.000 0.007 0.000 0.000 0.000 0.000 0.008 42 0.07 0.008 0.380.016 0.0057 0.0151 0.0047 0.000 0.920 0.000 0.000 0.000 0.000 0.008 430.08 0.010 0.36 0.017 0.0051 0.0148 0.0048 0.000 0.000 0.030 0.000 0.0000.000 0.008 44 0.08 0.009 0.44 0.017 0.0051 0.0153 0.0043 0.000 0.0002.880 0.000 0.000 0.000 0.008 45 0.06 0.011 0.42 0.018 0.0059 0.01520.0046 0.000 0.000 0.000 0.002 0.000 0.000 0.008 46 0.09 0.012 0.370.019 0.0053 0.0146 0.0048 0.000 0.000 0.000 1.920 0.000 0.000 0.008 470.06 0.012 0.37 0.018 0.0052 0.0147 0.0049 0.000 0.000 0.000 0.000 0.0020.000 0.008 48 0.05 0.009 0.43 0.015 0.0052 0.0147 0.0044 0.000 0.0000.000 0.000 1.980 0.000 0.008 49 0.05 0.012 0.44 0.018 0.0051 0.01490.0044 0.000 0.000 0.000 0.000 0.000 0.016 0.008 50 0.06 0.010 0.430.015 0.0052 0.0154 0.0046 0.000 0.000 0.000 0.000 0.000 0.140 0.008 510.05 0.010 0.43 0.018 0.0058 0.0148 0.0043 0.000 0.000 0.000 0.000 0.0000.000 0.012 52 0.05 0.012 0.42 0.019 0.0055 0.0150 0.0047 0.000 0.0000.000 0.000 0.000 0.000 0.130 53 0.05 0.011 0.40 0.016 0.0059 0.01530.0048 0.000 0.000 0.000 0.000 0.000 0.000 0.008 54 0.06 0.010 0.390.018 0.0051 0.0146 0.0049 0.000 0.000 0.000 0.000 0.000 0.000 0.008 550.09 0.012 0.37 0.017 0.0054 0.0152 0.0049 0.000 0.000 0.000 0.000 0.0000.000 0.008 56 0.07 0.012 0.40 0.018 0.0055 0.0153 0.0044 0.000 0.0000.000 0.000 0.000 0.000 0.008 57 0.06 0.008 0.41 0.015 0.0055 0.01460.0047 0.000 0.000 0.000 0.000 0.000 0.000 0.008 58 0.09 0.011 0.420.018 0.0058 0.0153 0.0045 0.000 0.000 0.000 0.000 0.000 0.000 0.008 590.08 0.010 0.38 0.014 0.0051 0.0230 0.0890 0.020 0.855 0.000 0.000 0.0000.000 0.009 Chemical Ingredients of Matrix Steel Sheet (in mass %.Balance is Fe and Impurities.) Ac₁ No. V B W Ca (° C.) Remark 31 0.00000.0003 0.000 0.000 747 32 0.0000 0.0003 0.000 0.000 713 33 0.0000 0.00030.000 0.000 742 34 0.0000 0.0003 0.000 0.000 742 35 0.0000 0.0003 0.0000.000 743 36 0.0000 0.0003 0.000 0.000 252 37 0.0000 0.0004 0.000 0.000760 38 0.0000 0.0004 0.000 0.000 760 39 0.0000 0.0003 0.000 0.000 742 400.0000 0.0003 0.000 0.000 812 41 0.0000 0.0003 0.000 0.000 742 42 0.00000.0003 0.000 0.000 762 43 0.0000 0.0003 0.000 0.000 741 44 0.0000 0.00030.000 0.000 675 45 0.0000 0.0003 0.000 0.000 742 46 0.0000 0.0003 0.0000.000 699 47 0.0000 0.0003 0.000 0.000 742 48 0.0000 0.0003 0.000 0.000742 49 0.0000 0.0003 0.000 0.000 746 50 0.0000 0.0003 0.000 0.000 772 510.0000 0.0003 0.000 0.000 742 52 0.0000 0.0003 0.000 0.000 741 53 0.00060.0003 0.000 0.000 742 54 0.9100 0.0003 0.000 0.000 707 55 0.0000 0.00070.000 0.000 741 56 0.0000 0.0092 0.000 0.000 734 57 0.0000 0.0003 0.9600.000 742 58 0.0000 0.0003 0.000 0.008 741 59 0.0000 0.0004 0.000 0.000760 Comparative steel

TABLE 2-1 Spherodizing annealing Continuous casting Nitrogen Soundnessenhancing Hot-rolling Cold-rolling concentration treatment of Finishrolling Winding Average Draft in in annealing Steel steel materialtemperature temperature Cooling start cooling rate cold-rollingatmosphere No. No. Yes/No (° C.) (° C.) time (s) (° C./s) (%) (%) 1 1 No868 595 0.7 99 — 84 2 2 No 884 596 0.8 57 — 76 3 3 No 856 593 0.7 80 —85 4 4 No 880 565 0.8 80 — 70 5 5 No 879 469 0.6 61 — 17 6 6 No 908 5040.4 99 — 19 7 7 No 851 585 0.4 64 — 83 8 8 No 871 608 0.5 99 — 72 9 9 No865 504 0.4 58 — 73 10 10 No 879 491 0.7 56 — 75 11 11 No 871 595 0.6 86— 71 12 12 No 874 610 0.7 99 — 82 13 13 No 852 489 0.5 98 — 81 14 14 No848 564 0.8 96 — 84 15 15 No 868 546 0.4 96 — 85 16 16 No 851 604 0.5 60— 79 17 17 No 857 598 0.4 96 — 74 18 18 No 877 502 0.7 56 — 80 19 19 No840 611 0.5 70 — 71 20 20 No 866 612 0.4 59 — 81 21 21 No 866 541 0.8 90— 73 22 22 No 848 599 0.6 70 — 84 23 23 No 841 544 0.5 61 — 81 24 24 No846 501 0.6 82 — 81 25 25 No 853 490 0.6 58 — 70 26 26 No 862 480 0.8 58— 85 27 27 No 874 582 0.8 59 — 72 28 28 No 867 515 0.6 87 — 70 29 2 No922 608 0.6 93 — 83 30 2 No 847 453 0.6 55 — 85 31 2 No 782 475 0.8 94 —85 32 2 No 851 756 0.6 64 — 80 33 2 No 871 560 0.7 84 — 75 Spherodizingannealing Average Heating Retention Average Steel heating ratetemperature time cooling rate Thickness No. No. (° C./h) (° C.) (h) (°C./h) (mm) Remark 1 1 16 659 66 45 5.5 Example 2 2 47 662 47 35 5.5Example 3 3 31 640 20 40 5.4 Example 4 4 50 648 78 43 4.6 Example 5 5 11731  4 11 5.2 Comparative Example 6 6 99 720 33 84 5.3 ComparativeExample 7 7 26 664 27 27 5.1 Comparative Example 8 8 48 659 45 20 5.3Example 9 9 27 645 60 27 5.5 Example 10 10 25 645 22 23 5.3 Example 1111 35 645 74 34 5.0 Comparative Example 12 12 29 661 71 44 5.7Comparative Example 13 13 20 684 38 34 4.6 Comparative Example 14 14 22665 34 31 5.1 Comparative Example 15 15 18 625 68 20 4.5 ComparativeExample 16 16 41 685 48 28 4.2 Example 17 17 24 675 60 19 5.6 Example 1818 31 611 66 19 4.3 Example 19 19 50 639 36 40 5.2 Example 20 20 20 65545 19 4.0 Example 21 21 24 682 50 48 4.6 Example 22 22 15 658 44 35 5.1Example 23 23 23 656 26 30 5.2 Example 24 24 28 661 63 31 5.6 Example 2525 28 661 23 43 5.4 Example 26 26 41 655 37 37 5.3 Example 27 27 19 65530 29 4.4 Example 28 28 24 668 47 26 5.5 Example 29 2 49 673 34 26 4.1Comparative Example 30 2 47 673 65 27 4.7 Example 31 2 34 653 62 18 4.5Comparative Example 32 2 32 663 27 28 4.9 Comparative Example 33 2 49661 60 35 5.7 Example

TABLE 2-2 Continuous casting Soundness Spherodizing annealing enhancingNitrogen treatment Hot-rolling Cold-rolling concentration of steelFinish rolling Winding Cooling Average Draft in in annealing Steelmaterial temperature temperature start cooling rate cold-rollingatmosphere No. No. Yes/No (° C.) (° C.) time (s) (° C./s) (%) (%) 34 2No 861 451 0.6 73 51 74 35 2 No 861 580 0.4 78 — 19 36 2 No 861 520 0.579 — 74 37 2 No 857 522 0.4 99 — 88 38 2 No 862 515 0.5 74 — 97 39 2 No860 450 0.6 55 — 78 40 2 No 883 583 0.7 80 — 83 41 2 No 882 553 0.4 56 —77 42 2 No 864 450 0.4 90 — 78 43 2 No 860 459 0.4 84 — 71 44 2 No 863575 0.5 68 — 84 45 2 No 876 553 0.6 100 — 77 46 2 No 883 535 0.4 79 — 8247 2 No 870 502 0.7 84 — 79 48 2 No 868 471 0.6 85 — 81 49 2 No 863 4870.4 63 — 85 50 2 No 867 518 0.6 97 — 80 51 2 Yes 865 501 0.8 65 — 84 5229 No 889 595 0.5 91 — 76 53 30 No 875 600 0.4 71 — 77 54 31 No 891 5920.5 95 — 79 55 32 No 880 597 0.4 57 — 81 56 33 No 889 603 0.4 91 — 72 5734 No 882 599 0.5 81 — 81 58 35 No 890 605 0.6 63 — 75 59 36 No 892 5940.7 94 — 81 60 37 No 894 606 0.8 67 — 76 61 38 No 882 598 0.5 55 — 80 6239 No 890 587 0.5 57 — 75 63 40 No 884 590 0.4 91 — 76 64 41 No 886 6060.7 96 — 71 65 42 No 876 593 0.6 69 — 72 66 43 No 879 602 0.4 64 — 75Spherodizing annealing Average Heating Retention Average Steel heatingrate temperature time cooling rate Thickness No. No. (° C./h) (° C.) (h)(° C./h) (mm) Remark 34 2 19 659 51 20 2.8 Example 35 2 33 661 67 49 4.5Comparative Example 36 2 38 679 72 49 4.6 Example 37 2 34 674 59 41 4.6Example 38 2 38 671 73 26 4.7 Example 39 2 130  666 55 37 4.8Comparative Example 40 2 45 664 35 39 4.2 Example 41 2  2 668 30 39 4.9Comparative Example 42 2 24 773 26 42 4.6 Comparative Example 43 2 37662 78 25 4.5 Example 44 2 41 617 43 44 4.7 Example 45 2 40 679 146  225.8 Comparative Example 46 2 20 662 24 41 5.3 Example 47 2 27 664  2 235.6 Comparative Example 48 2 48 659 31 130  4.5 Comparative Example 49 232 657 71 21 4.9 Example 50 2 44 668 22  2 5.4 Comparative Example 51 230 667 67 19 4.8 Example 52 29 51 667 43 30 5.6 Example 53 30 47 657 4832 5.3 Example 54 31 45 665 48 35 5.3 Example 55 32 55 660 42 36 5.5Example 56 33 38 663 46 35 5.6 Example 57 34 38 658 51 34 5.3 Example 5835 56 657 47 34 5.7 Example 59 36 57 662 42 33 5.3 Example 60 37 44 66248 40 5.3 Example 61 38 48 666 45 34 5.4 Example 62 39 44 658 47 38 5.3Example 63 40 40 658 52 37 5.6 Example 64 41 41 661 43 39 5.4 Example 6542 47 663 50 33 5.6 Example 66 43 39 666 44 35 5.3 Example

TABLE 2-3 Continuous casting Soundness enhancing Hot-rollingCold-rolling treatment of Finish rolling Winding Average Draft in Steelsteel material temperature temperature Cooling start cooling ratecold-rolling No. No. Yes/No (° C.) (° C.) time (s) (° C./s) (%) 67 44 No878 601 0.4 70 — 68 45 No 889 591 0.7 65 — 69 46 No 881 590 0.7 58 — 7047 No 887 597 0.5 75 — 71 48 No 881 587 0.4 59 — 72 49 No 886 592 0.4 83— 73 50 No 883 589 0.7 61 — 74 51 No 890 593 0.4 60 — 75 52 No 887 5900.4 74 — 76 53 No 890 598 0.6 80 — 77 54 No 879 604 0.7 60 — 78 55 No882 592 0.8 62 — 79 56 No 886 604 0.5 93 — 80 57 No 878 602 0.5 89 — 8158 No 893 593 0.7 56 — 82 2 No 809 606 0.5 67 — 83 2 No 877 691 0.8 61 —84 2 No 892 603 0.6 88 — 85 2 No 877 603 0.5 98 — 86 2 No 881 586 0.5 98— 87 2 No 881 593 0.7 55 — 88 2 No 894 597 0.4 83 — 89 2 No 893 587 0.798 — 90 2 No 893 605 0.5 75 — 91 2 No 895 612 1.8 74 — 92 2 No 880 5600.9 57 — 93 2 No 881 577 0.1 57 — 94 2 No 888 587 0.5 43 — 95 2 No 889588 0.8 151  — 96 2 No 876 591 0.8 203  — 97 2 No 884 596 0.8 57 — 98 59No 882 600 0.5 56 — Spherodizing annealing Nitrogen concentration inannealing Average Heating Retention Average Steel atmosphere heatingrate temperature time cooling rate Thickness No. No. (%) (° C./h) (° C.)(° C.) (° C./h) (mm) Remark 67 44 77 47 659 48 40 5.6 Example 68 45 7837 667 47 37 5.6 Example 69 46 79 40 659 42 39 5.4 Example 70 47 76 49662 51 31 5.3 Example 71 48 75 47 658 49 38 5.6 Example 72 49 75 47 65744 36 5.6 Example 73 50 80 43 662 45 40 5.3 Example 74 51 75 40 658 4736 5.7 Example 75 52 73 54 667 45 37 5.5 Example 76 53 76 40 661 47 325.4 Example 77 54 81 42 665 42 36 5.7 Example 78 55 80 57 657 44 31 5.5Example 79 56 80 52 661 52 39 5.3 Example 80 57 75 46 666 48 33 5.4Example 81 58 73 38 659 51 30 5.6 Example 82 2 81 53 659 47 30 5.7Example 83 2 79 38 665 43 30 5.5 Example 84 2 28 57 665 45 30 5.7Example 85 2 75 7 660 42 32 5.3 Example 86 2 74 94 664 52 32 5.6 Example87 2 73 55 660 12 38 5.5 Example 88 2 81 37 667 97 35 5.6 Example 89 273 53 666 46 8 5.5 Example 90 2 75 51 662 42 94 5.5 Example 91 2 77 48660 42 31 5.5 Comparative Example 92 2 77 37 651 48 30 5.7 Example 93 271 39 649 48 36 5.4 Example 94 2 70 39 643 40 30 5.5 Comparative Example95 2 75 42 653 47 34 5.6 Example 96 2 76 40 655 49 31 5.5 Example 97 296 47 660 47 35 5.5 Example 98 59 82 51 666 48 34 5.4 ComparativeExample

Each of the obtained steel sheets for carburizing was measured regarding(1) percentage of the number of carbides with an aspect ratio of 2.0 orsmaller, among from the total carbides, (2) percentage of the number ofcarbides produced in the ferrite crystal grains, among from the totalcarbides, (3) average equivalent circle diameter of carbides, (4)average nitrogen concentration in the skin layer of the steel sheet,and, (5) average crystal grain size of ferrite after spherodizingannealing, according to the methods described previously. Note that theaverage crystal grain size of ferrite after spherodizing annealing isunderstood to be the average crystal grain size of ferrite of theobtained steel sheet for carburizing.

In addition, in order to evaluate bendability of the obtained individualsteel sheets for carburizing, specimens were sampled from freelyselectable positions of the steel sheets for carburizing, and measuredregarding bendability under the following conditions, in compliance withthe VDA Standards (VDA238-100) specified by Verband derAutomobilindustrie e.V. In this test example, dislocation under maximumload obtainable in the bend test was converted to angle according to theVDA Standards, to determine maximum angle of bend (in degree).

-   Size of test specimen: 30 mm (rolling direction)×60 mm (direction    perpendicular to rolling direction)-   Bending ridge: laid in parallel with rolling direction-   Test method: roll-supported, punch-pressed-   Roll diameter: φ 30 mm-   Punch shape: end with R=0.4 mm-   Roll-to-roll distance: 2.0×sheet thickness (mm)+0.5 mm-   Pressing velocity: 20 mm/min-   Tester: Shimadzu Autograph (registered trademark) 20 kN

Also in order to evaluate toughness after carburizing of the obtainedindividual steel sheets for carburizing, each of the thus obtained steelsheets for carburizing was carburized as described below. That is, eachof the steel sheets for carburizing was carburized while being kept in agas atmosphere with a carbon potential of 0.8 mass % at 900° C. for 2.5hr, and further being kept at 850° C. for 0.5 hr, and then oil-quenchedat 100° C. The steel sheet was then kept at 160° C. for 2.0 hr fortempering, and cooled down to room temperature. A 2 mm V-notched Charpytest piece was sampled from a freely selectable position of the steelsheet after carburizing heat treatment, and subjected to Charpy test atroom temperature in compliance with a method specified in JIS Z2242, tomeasure the impact value (J/cm²).

As a reference, also ideal critical diameter, which is an index forhardenability after carburizing, was calculated. The ideal criticaldiameter D_(i) is an index calculated from ingredients of the steelsheet, and may be determined using the equation (201) according toGrossmann/Hollomon, Jaffe's method. The larger the value of idealcritical diameter D, the more excellent the hardenability.

$\begin{matrix}\left\lbrack {{Math}.\mspace{14mu} 3} \right\rbrack & \; \\{{D_{i} = {\left( {6.77 \times \lbrack C\rbrack^{0.5}} \right) \times \left( {1 + {0.64 \times \left\lbrack {Si} \right\rbrack}} \right) \times \left( {1 + {4.1 \times \lbrack{Mn}\rbrack}} \right) \times \left( {1 + {2.83 \times \lbrack P\rbrack}} \right) \times \left( {1 - {0.62 \times \lbrack S\rbrack}} \right) \times \left( {1 + {0.27 \times \lbrack{Cu}\rbrack}} \right) \times \left( {1 + {0.52 \times \lbrack{Ni}\rbrack}} \right) \times \left( {1 + {2.33 \times \lbrack{Cr}\rbrack}} \right) \times \left( {1 + {3.14 \times \lbrack{Mo}\rbrack}} \right) \times X}}\mspace{275mu} {{{For}\mspace{14mu}\lbrack B\rbrack} = {{0:X} = 1}} {{{{{For}\mspace{14mu}\lbrack B\rbrack} > 0}:X} = {1 + {1.5 \times \left( {0.9 - \lbrack C\rbrack} \right)}}}} & {{Equation}\mspace{14mu} (201)}\end{matrix}$

In this test example, the cases where the maximum bending angle of thesteel sheet for carburizing is 100° or larger, and the impact valueafter carburizing is 60 J/cm² or larger were judged to show highbendability during cold-working and high toughness after carburizing,and were accepted as “examples”.

Microstructures and characteristics of the individual steel sheets forcarburizing thus obtained were collectively summarized in Table 3 below.

TABLE 3-1 Microstructure Average nitrogen Average circle concentrationPercentage of number Percentage of number equivalent Average crystalgrain in skin layer of carbides with aspect of carbides within diameterof size of ferrite after Steel of steel sheet ratio of 2.0 or smallerferrite crystal grain carbide spherodizing annealing No. No. (mass %)(%) (%) (μm) (μm) 1 2 0.051 97 75 0.54 7.7 2 2 0.056 97 81 0.61 7.0 3 30.051 86 83 0.47 7.1 4 4 0.045 91 87 0.60 6.8 5 5 0.006 89 31 0.46 4.8 66 0.007 91 89 0.56 7.2 7 7 0.051 82 45 0.61 4.1 8 8 0.054 92 69 0.35 5.59 9 0.053 86 68 0.36 6.0 10 10 0.054 85 86 0.45 4.0 11 11 0.060 84 687.58 7.0 12 12 0.058 85 86 6.85 6.6 13 13 0.051 96 68 6.46 8.0 14 140.056 97 87 6.95 5.6 15 15 0.050 90 83 6.99 8.0 16 16 0.055 84 77 0.494.5 17 17 0.049 94 74 0.37 5.3 18 18 0.049 88 89 0.57 6.4 19 19 0.048 9283 0.58 6.0 20 20 0.051 96 82 0.65 6.1 21 21 0.054 84 78 0.59 4.2 22 220.049 83 71 0.47 4.3 23 23 0.045 90 76 0.52 7.5 24 24 0.061 97 78 0.426.1 25 25 0.061 86 88 0.39 5.4 26 26 0.056 89 86 0.42 4.0 27 27 0.053 8268 0.48 6.2 28 28 0.053 90 78 0.53 4.7 29 2 0.058 49 78 0.35 4.4 30 20.047 84 80 0.35 4.8 31 2 0.060 98 73 7.25 4.3 32 2 0.047 66 69 0.55 7.233 2 0.045 84 89 0.59 5.5 Mechanical characteristics Maximum Impactvalue Hardenability bending after Ideal critical Steel angle carburizingdiameter No. No. (deg) (J/cm²) (—) Remark 1 2 120 82 20.7 Example 2 2113 85 43.9 Example 3 3 108 66 23.2 Example 4 4 120 78 135.1 Example 5 5 88 81 20.5 Comparative Example 6 6  72 64 108.5 Comparative Example 7 7 69 63 1.5 Comparative Example 8 8 105 64 9.7 Example 9 9 106 66 9.5Example 10 10 101 71 9.6 Example 11 11  79 67 13.9 Comparative Example12 12  67 85 5.3 Comparative Example 13 13  69 83 12.6 ComparativeExample 14 14  77 83 1.8 Comparative Example 15 15  85 83 31.4Comparative Example 16 16 114 80 23.5 Example 17 17 118 75 15.7 Example18 18 113 82 7.8 Example 19 19 115 84 7.8 Example 20 20 111 80 5.6Example 21 21 113 76 5.6 Example 22 22 120 83 5.6 Example 23 23 111 805.9 Example 24 24 117 81 13.5 Example 25 25 110 82 5.9 Example 26 26 11681 6.4 Example 27 27 114 85 5.8 Example 28 28 113 82 5.2 Example 29 2 77 81 43.9 Comparative Example 30 2 110 81 43.9 Example 31 2  79 8043.9 Comparative Example 32 2  86 85 43.9 Comparative Example 33 2 11982 43.9 Example

TABLE 3-2 Microstructure Average nitrogen Average circle concentrationPercentage of number Percentage of number equivalent Average crystalgrain in skin layer of carbides with aspect of carbides within diameterof size of ferrite after Steel of steel sheet ratio of 2.0 or smallerferrite crystal grain carbide spherodizing annealing No. No. (mass %)(%) (%) (μm) (μm) 34 2 0.047 85 73 0.58 4.5 35 2 0.007 96 84 6.37 7.0 362 0.055 86 80 0.35 4.6 37 2 0.188 86 80 0.35 6.9 38 2 0.181 86 80 0.355.0 39 2 0.057 41 75 0.51 7.1 40 2 0.048 90 71 0.54 7.5 41 2 0.047 92 726.69 6.5 42 2 0.055 85 36 0.43 6.8 43 2 0.057 96 85 0.58 5.7 44 2 0.06196 78 0.57 7.9 45 2 0.056 88 68 5.95 7.2 46 2 0.055 92 86 0.59 7.2 47 20.050 65 70 0.48 5.0 48 2 0.049 71 68 0.49 5.9 49 2 0.048 95 82 0.42 7.250 2 0.054 88 79 7.61 5.0 51 2 0.055 96 81 0.41 5.9 52 29 0.056 88 750.63 5.6 53 30 0.050 90 80 4.70 7.7 54 31 0.061 91 80 0.71 5.9 55 320.050 88 77 4.81 7.8 56 33 0.064 91 78 0.64 4.3 57 34 0.059 90 77 0.517.5 58 35 0.061 88 74 0.65 7.2 59 36 0.046 87 77 0.62 5.9 60 37 0.057 8772 0.62 7.8 61 38 0.052 89 82 0.58 5.8 62 39 0.061 92 81 0.67 9.5 63 400.056 90 79 0.56 5.1 64 41 0.061 92 73 0.60 9.7 65 42 0.047 92 81 0.668.0 66 43 0.066 89 76 0.55 9.1 Mechanical characteristics Maximum Impactvalue Hardenability bending after Ideal critical Steel angle carburizingdiameter No. No. (deg) (J/cm²) (—) Remark 34 2 110 82 43.9 Example 35 2 71 55 43.9 Comparative Example 36 2 114 85 43.9 Example 37 2 117 9443.9 Example 38 2 113 87 43.9 Example 39 2  81 76 43.9 ComparativeExample 40 2 111 79 43.9 Example 41 2  86 75 43.9 Comparative Example 422  78 80 43.9 Comparative Example 43 2 113 79 43.9 Example 44 2 112 8343.9 Example 45 2  87 85 43.9 Comparative Example 46 2 117 77 43.9Example 47 2  71 85 43.9 Comparative Example 48 2  74 78 43.9Comparative Example 49 2 116 84 43.9 Example 50 2  82 79 43.9Comparative Example 51 2 131 95 43.9 Example 52 29 103 90 11.4 Example53 30 102 89 14.4 Example 54 31 104 81 5.9 Example 55 32 101 76 60.3Example 56 33 105 78 12.2 Example 57 34 101 87 10.7 Example 58 35 102 8012.9 Example 59 36 104 85 9.8 Example 60 37 128 82 43.9 Example 61 38102 81 43.9 Example 62 39 114 64 9.9 Example 63 40 115 71 101.6 Example64 41 113 62 11.4 Example 65 42 116 70 41.9 Example 66 43 110 63 11.3Example

TABLE 3-3 Microstructure Average nitrogen Average circle concentrationPercentage of number Percentage of number equivalent Average crystalgrain in skin layer of carbides with aspect of carbides within diameterof size of ferrite after Steel of steel sheet ratio of 2.0 or smallerferrite crystal grain carbide spherodizing annealing No. No. (mass %)(%) (%) (μm) (μm) 67 44 0.050 90 77 0.58 4.6 68 45 0.059 91 80 0.64 9.469 46 0.058 90 73 0.57 4.1 70 47 0.050 92 78 0.70 9.4 71 48 0.064 92 800.66 6.1 72 49 0.059 92 76 0.55 9.1 73 50 0.059 91 77 0.55 5.1 74 510.064 90 78 0.51 9.3 75 52 0.055 91 83 0.57 4.0 76 53 0.055 90 74 0.639.5 77 54 0.056 91 75 0.63 7.0 78 55 0.066 91 79 0.64 6.4 79 56 0.052 9081 0.61 6.6 80 57 0.058 89 92 0.70 5.1 81 58 0.046 88 80 0.69 7.6 82 20.052 87 75 4.66 6.2 83 2 0.058 81 75 0.68 6.8 84 2 0.041 92 62 0.60 5.485 2 0.066 90 79 4.71 4.8 86 2 0.052 82 77 0.63 5.5 87 2 0.046 81 710.69 7.6 88 2 0.057 91 82 4.81 6.8 89 2 0.048 92 71 4.71 4.6 90 2 0.05781 73 0.63 6.4 91 2 0.064 92 71 0.71 13.0  92 2 0.066 92 79 0.54 9.1 932 0.054 89 81 0.56 2.3 94 2 0.060 92 73 0.64 12.5  95 2 0.058 89 79 0.603.2 96 2 0.065 92 77 0.68 1.8 97 2 0.050 91 76 0.70 4.1 98 59 0.289 8981 0.54 5.7 Mechanical characteristics Maximum Impact valueHardenability bending after Ideal critical Steel angle carburizingdiameter No. No. (deg) (J/cm²) (—) Remark 67 44 111 65 31.4 Example 6845 117 63 10.8 Example 69 46 116 62 18.2 Example 70 47 118 65 10.0Example 71 48 115 62 9.9 Example 72 49 101 81 10.2 Example 73 50 103 8010.8 Example 74 51 103 87 10.0 Example 75 52 105 84 9.9 Example 76 53103 78 9.5 Example 77 54 104 76 10.3 Example 78 55 102 81 11.9 Example79 56 125 75 11.2 Example 80 57 121 84 10.5 Example 81 58 118 78 12.9Example 82 2 103 83 43.9 Example 83 2 101 79 43.9 Example 84 2 110 6243.9 Example 85 2 103 76 43.9 Example 86 2 104 79 43.9 Example 87 2 10388 43.9 Example 88 2 101 83 43.9 Example 89 2 101 85 43.9 Example 90 2102 76 43.9 Example 91 2 110 51 43.9 Comparative Example 92 2 118 6243.9 Example 93 2 118 93 43.9 Example 94 2 107 52 43.9 ComparativeExample 95 2 108 91 43.9 Example 96 2 109 97 43.9 Example 97 2 114 9943.9 Example 98 59  54 82 43.9 Comparative Example

As is clear from Table 3 above, the steel sheets for carburizing thatcorrespond to the examples of this invention were found to have goodformability and toughness after carburizing, showing maximum bendingangles of the steel sheet for carburizing of 100° or larger, and impactvalues after carburizing of 60 J/cm² or larger. Also the ideal criticaldiameter, described for reference, was found to be 5 or larger, teachingthat the steel sheets for carburizing that come under examples of thepresent invention also excel in hardenability.

Meanwhile, as is clear from Table 3 above, the steel sheets forcarburizing that correspond to comparative examples of this inventionwere found to be ill-balanced between the formability and the toughnessafter carburizing, showing at least either of maximum bending angle orimpact value after carburizing dropped below the standard values.

Although having detailed the preferred embodiments of the presentinvention, the present invention is not limited to these examples. It isobvious that those having general knowledge in the technical field towhich the present invention pertains will easily arrive at variousmodified examples or revised examples within the scope of technicalconcept described in claims, and also these examples are naturallyunderstood to come under the technical scope of the present invention.

1. A steel sheet for carburizing consisting of, in mass %, C: more thanor equal to 0.02%, and less than 0.30%, Si: more than or equal to0.005%, and less than or equal to 0.5%, Mn: more than or equal to 0.01%,and less than or equal to 3.0%, P: less than or equal to 0.1%, S: lessthan or equal to 0.1%, sol. Al: more than or equal to 0.0002%, and lessthan or equal to 3.0%, N: more than or equal to 0.0001%, and less thanor equal to 0.035%, and the balance: Fe and impurities, wherein averagecrystal grain size of ferrite is smaller than 10 average equivalentcircle diameter of carbide is 5.0 μm or smaller, percentage of number ofcarbides with an aspect ratio of 2.0 or smaller is 80% or largerrelative to the total carbides, percentage of number of carbides presentin ferrite crystal grain is 60% or larger relative to the totalcarbides, and average nitrogen concentration in a region ranging fromtopmost surface of steel sheet to a depth of 50 μm is 0.040 mass % orhigher and 0.200 mass % or lower.
 2. The steel sheet for carburizingaccording to claim 1, further comprising, in place of part of thebalance Fe, one of, or two or more of, in mass %, Cr: more than or equalto 0.005%, and less than or equal to 3.0%, Mo: more than or equal to0.005%, and less than or equal to 1.0%, Ni: more than or equal to0.010%, and less than or equal to 3.0%, Cu: more than or equal to0.001%, and less than or equal to 2.0%, Co: more than or equal to0.001%, and less than or equal to 2.0%, Nb: more than or equal to0.010%, and less than or equal to 0.150%, Ti: more than or equal to0.010%, and less than or equal to 0.150%, V: more than or equal to0.0005%, and less than or equal to 1.0%, and B: more than or equal to0.0005%, and less than or equal to 0.01%.
 3. The steel sheet forcarburizing according to claim 1, further comprising, in place of partof the balance Fe, at least either one of, in mass %, W: less than orequal to 1.0%, or Ca: less than or equal to 0.01%.
 4. A method formanufacturing the steel sheet for carburizing according to claim 1, themethod comprising: a hot-rolling step, in which a steel material havingthe chemical composition of said steel sheet is heated, hot finishrolling is terminated in a temperature range of 800° C. or higher andlower than 920° C., followed by winding at a temperature of 700° C. orlower; and an annealing step, in which the steel sheet obtained by thehot-rolling step, or, the steel sheet having been cold-rolledsubsequently to the hot-rolling step is heated in an atmosphere withnitrogen concentration controlled to 25% or higher in volume fraction,at an average heating rate of 5° C./h or higher and 100° C./h or lower,up into a temperature range not higher than point Ac₁ defined byequation (1) below, annealed in the temperature range not higher thanthe point Ac₁ for 10 h or longer and 100 h or shorter, and then cooledat an average cooling rate of 5° C./h or higher and 100° C./h or lowerin a temperature range from a temperature at the end of annealing downto 550° C., in the hot-rolling step, cooling being started within onesecond after end of the hot finish rolling, at an average cooling rateof higher than 50° C./s, and an average grain size of ferrite after theannealing being controlled to smaller than 10 μm, where in equation (1)below, notation [X] represents the content of element X (in mass %),which is substituted by zero if such element X is absent,$\begin{matrix}{{Ac}_{1} = {750.8 - {26.6\lbrack C\rbrack} + {17.6\lbrack{Si}\rbrack} - {11.6\lbrack{Mn}\rbrack} - {22.9\lbrack{Cu}\rbrack} - {23\lbrack{Ni}\rbrack} + {24.1\lbrack{Cr}\rbrack} + {22.5\lbrack{Mo}\rbrack} - {39.7\lbrack V\rbrack} - {5.7\lbrack{Ti}\rbrack} + {232.4\lbrack{Nb}\rbrack} - {169.4\lbrack{Al}\rbrack} - {{894.7\lbrack B\rbrack}.}}} & {{Equation}\mspace{14mu} (1)}\end{matrix}$
 5. A method for manufacturing the steel sheet forcarburizing according to claim 4, further comprising: a continuouscasting step for obtaining the steel material to be subjected to thehot-rolling step, in which at least either soundness enhancing treatmentof the steel material, namely production of a predetermined inclusion,or reduction of center segregation of a predetermined element, iscarried out.
 6. A steel sheet for carburizing comprising, in mass %, C:more than or equal to 0.02%, and less than 0.30%, Si: more than or equalto 0.005%, and less than or equal to 0.5%, Mn: more than or equal to0.01%, and less than or equal to 3.0%, P: less than or equal to 0.1%, S:less than or equal to 0.1%, sol. Al: more than or equal to 0.0002%, andless than or equal to 3.0%, N: more than or equal to 0.0001%, and lessthan or equal to 0.035%, and the balance comprising Fe and impurities,wherein average crystal grain size of ferrite is smaller than 10 μm,average equivalent circle diameter of carbide is 5.0 μm or smaller,percentage of number of carbides with an aspect ratio of 2.0 or smalleris 80% or larger relative to the total carbides, percentage of number ofcarbides present in ferrite crystal grain is 60% or larger relative tothe total carbides, and average nitrogen concentration in a regionranging from topmost surface of steel sheet to a depth of 50 μm is 0.040mass % or higher and 0.200 mass % or lower.
 7. The steel sheet forcarburizing according to claim 6, further comprising, in place of partof the balance Fe, one of, or two or more of, in mass %, Cr: more thanor equal to 0.005%, and less than or equal to 3.0%, Mo: more than orequal to 0.005%, and less than or equal to 1.0%, Ni: more than or equalto 0.010%, and less than or equal to 3.0%, Cu: more than or equal to0.001%, and less than or equal to 2.0%, Co: more than or equal to0.001%, and less than or equal to 2.0%, Nb: more than or equal to0.010%, and less than or equal to 0.150%, Ti: more than or equal to0.010%, and less than or equal to 0.150%, V: more than or equal to0.0005%, and less than or equal to 1.0%, and B: more than or equal to0.0005%, and less than or equal to 0.01%.
 8. The steel sheet forcarburizing according to claim 6, further comprising, in place of partof the balance Fe, at least either one of, in mass %, W: less than orequal to 1.0%, or Ca: less than or equal to 0.01%.
 9. A method formanufacturing the steel sheet for carburizing according to claim 6, themethod comprising: a hot-rolling step, in which said steel materialhaving the chemical composition of said steel sheet is heated, hotfinish rolling is terminated in a temperature range of 800° C. or higherand lower than 920° C., followed by winding at a temperature of 700° C.or lower; and an annealing step, in which the steel sheet obtained bythe hot-rolling step, or, the steel sheet having been cold-rolledsubsequently to the hot-rolling step is heated in an atmosphere withnitrogen concentration controlled to 25% or higher in volume fraction,at an average heating rate of 5° C./h or higher and 100° C./h or lower,up into a temperature range not higher than point Ac₁ defined byequation (1) below, annealed in the temperature range not higher thanthe point Ac₁ for 10 h or longer and 100 h or shorter, and then cooledat an average cooling rate of 5° C./h or higher and 100° C./h or lowerin a temperature range from a temperature at the end of annealing downto 550° C., in the hot-rolling step, cooling being started within onesecond after end of the hot finish rolling, at an average cooling rateof higher than 50° C./s, and an average grain size of ferrite after theannealing being controlled to smaller than 10 μm, where in equation (1)below, notation [X] represents the content of element X (in mass %),which is substituted by zero if such element X is absent,$\begin{matrix}{{Ac}_{1} = {750.8 - {26.6\lbrack C\rbrack} + {17.6\lbrack{Si}\rbrack} - {11.6\lbrack{Mn}\rbrack} - {22.9\lbrack{Cu}\rbrack} - {23\lbrack{Ni}\rbrack} + {24.1\lbrack{Cr}\rbrack} + {22.5\lbrack{Mo}\rbrack} - {39.7\lbrack V\rbrack} - {5.7\lbrack{Ti}\rbrack} + {232.4\lbrack{Nb}\rbrack} - {169.4\lbrack{Al}\rbrack} - {{894.7\lbrack B\rbrack}.}}} & {{Equation}\mspace{14mu} (1)}\end{matrix}$
 10. A method for manufacturing the steel sheet forcarburizing according to claim 9, further comprising: a continuouscasting step for obtaining the steel material to be subjected to thehot-rolling step, in which at least either soundness enhancing treatmentof the steel material, namely production of a predetermined inclusion,or reduction of center segregation of a predetermined element, iscarried out.